Mixed Blessings for the Clippers : Basketball: They agree with Mark Jackson on a five-year contract, but can’t hold off the Trail Blazers and lose, 101-99.

PORTLAND, Ore. — The Clippers took a significant step for the long-term future Tuesday by reaching an agreement in principle with guard Mark Jackson on a five-year, $13-million contract extension.

But their try at a significant step for the short term crumbled under the weight of a scoreless final 3:12, resulting in a 101-99 loss to the Portland Trail Blazers and a missed opportunity to all but wrap up seventh place in the Western Conference standings.

A victory at Memorial Coliseum would have given the Clippers a three-game lead, plus the tiebreaker edge, over the Lakers with five games remaining. Instead, with the last of Terry Porter’s 26 points coming on a three-point basket with 22.5 seconds left, they must wait.

“They stopped us every trip,” Clipper Coach Larry Brown said. “I don’t know how many times we got a shot but didn’t get it to go in.”

Five times the final 3:12, after Ron Harper’s two free throws. The Clippers’ last field goal came with 5:46 remaining.

The Clippers trailed, 81-78, entering the fourth quarter, but scored on seven consecutive trips to take a 92-86 lead. When Portland closed the gap to 92-91 on a five-point possession--two free throws by Porter on a flagrant foul and then a three-point baseline jumper by Porter--the Trail Blazers grabbed the momentum back.

They next grabbed the lead back, first at 98-97 with 3:26 left, then for the game at 101-99 on Porter’s baseline three-pointer over a charging Ken Norman. It was the Trail Blazers’ first field goal since 6:44.

The Clippers had two shots for the tie the last nine seconds, but one attempt by Danny Manning was blocked by Cliff Robinson and an off-balance runner by Harper rolled around the rim and fell out. A final try, Jackson’s 18-footer at the buzzer, would not have counted.

The Trail Blazers had scored 110 points in three of the previous four games without Clyde Drexler, though two of the opponents were Dallas and Minnesota, and then hit the Clippers for 62 in the first half. That was worth a nine-point lead for Portland, which doesn’t figure to get Drexler back from the injured list with a hamstring problem until two or three games remain in the regular season.

The Clippers hope to make an announcement on Jackson’s deal as soon as today. That’s when the parties are expected to sign the contract that starts next season and is guaranteed through 1997-98 at an average of $2.6 million per season.

“I’m really happy to get the negotiations over with and to be part of the organization for another five years,” said Jackson, who makes $1.75 million this season and would have become a restricted free agent July 1. It’s going to be great. When I look at how things were going for me two years ago, three years ago, this is a long step I’ve made. I’m proud and I’m honored.

“It’s great because this is where I wanted to be since Day 1. Coach Brown, the way he has accepted me and our relationship has developed, and my teammates have made this a great situation.”

Said Brown: “I’m obviously thrilled. He’s been a great addition to our club and has had a great year. And he is a tremendous person, a guy you’d like to have on your club for years to come.”

The Clippers acquired Jackson from the New York Knicks, where he spent an up-and-down five seasons, in September as part of the three-team trade that also brought Stanley Roberts from Orlando. The 28-year-old former rookie of the year has responded by starting all 77 games.

He went into the game against the Trail Blazers fourth in the league in assists, at 9.1 a game, and is on pace to break the team record for assists and minutes.

Jackson’s impending signing leaves the Clippers with four potential free agents among their core: Norman will become unrestricted in the summer, Harper will be unrestricted if the team chooses not to pick up his $4-million option for next season by June 15, and Manning and Gary Grant will be restricted.

Harper and Norman are the next priorities, though Norman, who has turned down an offer of $10 million for four years, said he does not want to talk until the end of the season. Manning would jump to the top of the list, but he is not entertaining Clipper offers.