Longman Dictionary of Contemporary English
The Collaborative International Dictionary
first-string \first-string\ adj. First to play in a game; not reserved as a substitute; -- of members of a team. Also used in non-sports contexts to mean first-rate.
Wiktionary
a. describing the members of a sports team who play regularly at the start of a match (rather than being substituted on)
WordNet
adj. of members of a team; not substitutes
of the regular members of a team; not substitutes; "first-string players"
Usage examples of "first-string".
Our team was far behind-we had been substituting second-string players for first-string players so often and so randomly that I could no longer recognize half of our own batters-and I had lost track of my place in the batting order.
The Reverend caught him boosting a pack of gum at the local Pig and Whistle and flayed his ass so bad that Big Des popped a bunch of tendons trying to get free of his bonds and was sidelined for the rest of the football season, a first-string linebacker with a third-string brain and a first-class case of kleptomania that he was now terrified to run with: no legs and no balls, courtesy of Liam Considine, first-string Calvinist.
In the season finale, they crushed Dartmouth, their traditional rival, 33-0, Chris Jastrow was named first-string Ivy quarterback and looked likely to be drafted by the pros.