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Longman Dictionary of Contemporary English
first-string
adjective
PHRASES FROM OTHER ENTRIES
first-string/second-string etc
EXAMPLES FROM CORPUS
▪ 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.
The Collaborative International Dictionary
first-string

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
first-string

a. describing the members of a sports team who play regularly at the start of a match (rather than being substituted on)

WordNet
first-string
  1. adj. of members of a team; not substitutes

  2. 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.