Bill Sibbick, a 21-year-old University of North Carolina senior, added a fillip to a fine day of golf with a hole in one in his afternoon round to gain the final against Neff McClary of Fairfax in the Virginia State Amateur Golf Championship at Washington Golf and Country Club yesterday. They will meet in a 36-hole showdown today, starting at 8 a.m.

Sibbick disposed of one of the University of Maryland's young golfers. Mark Lawrence, 5 and 4, in one semifinal while McClary took care of Eric Smith, another Terrapin golfer, in the other, 1 up.

McClary gained the semifinals by beating Marty Alford, former Naval Academy golf caption, 3 and 2, when Alford, who was 2 up, bogeyed five of the last six holes. Smith ousted John Long of Manassas, 1 up, in the morning. It marked the third of the four matches Smith had won by 1 up, but, ironically, he lost by the same margin in the afternoon.

Sibbick defeated Marvin Smith of Richmond, 3 and 2, in the morning and Lawrence eliminated Tim Baker of the University of South Carolina, 5 and 4.

Sibbick was up on Lawrence at the turn in his afternoon match, and then birdied the 10th when he put a wedge shot five feet from the hole to go 2 up.

Then came the shot on the 196-yard 11th that killed any thoughts Lawrence may have had of coming back. Sibbick used a four-iron and the bail landed a foot in front of the hole, dug a tiny crater in the green, spun out and rolled into the cup for an ace. Still hot, Sibbick won the 12th ad 13th with pars when the 18-year-old Lawrence three-putted each time. The matchended on the short 14th, which was halved with par threes. Sibbick was four under par for 14 holes.

McClary, 33, who is in the construction business with his father in Alexandria, won the District Amateur in 1965. He carries a pocketful of cigars when he is playing and has one constantly in his mouth.

Smith wanted to celebrate his 19th birthday today by getting into the final round but the veteran McClary had other ideas. Smith birdied the first hole and parred the second to take a quick 2-up advantage. But McClary won the fifth with a par and squared the match with another par on the seventh.

Smith birdied the ninth from 15 feet to take a 1-up advantage at the turn. But McClary leveled it again with a par on the 10th when Smith made a bad chip shot and bogeyed.

The deciding hole was the 13th when McClary rolled in a 20-footer for a birdie three. The next five holes were halved although McClary had to make a tricky four-footer on the 18th for his par after coming out from under the trees on his second shot.