Leslie Combs II, who revolutionized the thoroughbred industry with the syndication of horses and built his Spendthrift Farm into one of the giants of the industry, is selling controlling interest in the farm. Combs and his son Brownell Combs II own 63.7 percent of more than 13 million shares of Spendthrift Farm Inc., which owns the 2,450-acre farm near Lexington, Ky . . .

A crackdown on illegal aliens employed as backstretch workers at Del Mar race track in California has prompted an exodus of Hispanic stablehands, and left trainers in charge of expensive thoroughbreds scrambling for help. As many as 300 grooms, exercise riders and handlers fled their jobs at the track during the weekend after the U.S. Border Patrol threatened to raid the backstretch, said Joseph Harper, the track's executive vice president and general manager.

Hundreds of others were said to have their bags packed, ready to leave in case of a raid. According to the Border Patrol, as many as 50 percent of the 3,000 backstretch workers at the track are illegal aliens.

Track officials were meeting with immigration officers in an attempt to ward off the exodus, which could cause cancellation of the meet . . .

At least five horses were killed when a barn caught fire last night at the Red Mile harness track in Lexington, Ky. Among the horses killed was Banker Barker, a 7-year-old gelding who was the top male trotter of 1981.