The Los Angeles Raiders today signed free agent lineman Dale Hellestrae, who finished the 1988 NFL season as a starting guard for the AFC East champion Buffalo Bills. Hellestrae, 26, was left on the Bills' unprotected list when the NFL's new free-agency system went into operation Feb. 1. Hellestrae, a 6-foot-5 285-pounder, was a fourth-round draft choice of the Bills in 1985 after he completed his college career at Southern Methodist. He was a reserve interior lineman and long snapper for Buffalo until earning a starting berth last season. Among the games started by Hellestrae was one Dec. 11 when the Bills beat the Raiders 37-21 at Buffalo.