Exceller was the winner of yesterday's Jockey Club Gold Cup, Seattle Slew was its hero.

Slew was subjected to an impossible challenge at Belmont Park, and he responded as only a great horse could. He battled head and head with the tenacious Affirmed at a pace that would have been suicidal for most any thoroughbred. When the stretch-running Exceller started to sweep past him, Slew battled him with unyielding determination.

The fact that he failed by the margin of a nose barely detracted from his achievement. The other prominent loser in the field, Affirmed, emerged from the race with an excuse, if not with glory; his saddle slipped as he raced down the backstretch, eliminating any chance he might have had.

The 60th running of the Gold Cup had appeared to be a race that would be decided by tactics - and that assessment proved to be correct. Laz Barera, the trainer of Affirmed, knew that he could not afford to let Slew take an early lead.

Slew had done just that in the Marlboro Cup a month earlier, so Barera entered Life's Hope as a pacesetter.

This proved to be superfluous. Slew came out of the gate in high gear, as expected, and Life's Hope ran with him, as expected. But Affirmed was battling them instead of laying off the pace; jockey Steve Cauthen wasn't urging him, but colt is a battler by nature and he could not resist the challenge.

The horses sped three-abreast around the turn, covering a quarter mile in 22 3/5 seconds and a half mile in 45 1/5, a pace more appropriate for a sprint than the stamina-testing distance of a mile and one half.

The pace had to worry the fans of Seattle Slew, who in his three-year career had often shown dazzling speed but had never been tested for competiveness, for that elusive race track quality called "class." The pace surely worried Barrera, who had predicted before the Gold Cup that a suicidal duel between the two speedy horses would set the stage for a stretch charge by Exceller.

While the battle was transpiring on the front end, 47-year-old Willie Shoemaker was biding his time aboad Exceller. "I couldn't even see the leaders I was so far back," he said. But he knew the horses would have to come back to him eventually. And they did, one by one.

First Life's Hope collapsed. Then, after three-quarters of a mile, Affirmed started to drop back, as Cauthen stood up in the saddle several times, with his saddle slipping. Slew took a clear lead, and the crowd of 25,018 let out a roar. But they were yelling not at the front-runner but at the green silks that had just come into binocular range and were now moving at high velocity.

Exceller was gliding through the slop which he supposedly did not like, and when he drew abreast of Seattle Slew with less than a quarter mile to go, every spectator must have assumed the race was over. So did Shoemaker. "I thought we'd win it easily," he said. But jockey Angel Cordero could feel Slew responding to the challenge. "He wouldn't give up," Cordero said.

Even after Exceller had opened a half-length lead in midstretch, Slew refusal to yield, and battled back to cut his deficit to approximately one inch at the finish line. The horses had covered the mile and one-half in an excellent 2:27 1/5.

Although Exceller lacks the reputation of Slew and Affirmed, who got great media exposure in their sweeps of the Triple Crown, he came into the race with formidable credentials of his own, enough for the cognoscenti at Belmont to back his odds down to 7 to 2. He paid $9.69, $3 and $2.80. Slew paid $2.40 and $2.20. Exceller had won major stakes in England, France, Canada and the United States, and his share of the $321,800 purse makes him the fifth-leading money winner of all time.

The victory also will give him a plausible claim to the Horse of the Year crown. But Seattle Slew staked a greater claim to the title in defeat than the ever had in victory.

Affirmed, this year's Triple Crown winner, finished fifth behind Great Contractor and One Cut above and more than 18 lengths back of the winner.

Great Contractor, winner this race in 1976, paid $3.20 after finishing 14 1/2 lengths behind Seattle Slew and 4 3/4 lengths ahead of One Cut Above.

lined for several months with a back injury when his mount fell heavily while making a strong move on the stretch turn.