From Longman Dictionary of Contemporary Englishfirst-stringˌfirst-ˈstring adjective [only before noun] DSa first-string player in a team plays when the game begins because they are the most skilled → second-string
Examples from the Corpus
first-string• That holder was the first-string quarterback, so Stewart had created a job opening.• They also have incurred some important injuries, including the loss of first-string quarterback Steve Young for several games. 