second string is a noun meaning "a squad of players that are available either individually or as a team to relieve or replace the players who started the game". It carries one recorded sense.
a squad of players that are available either individually or as a team to relieve or replace the players who started the game
Definitions, synonyms and semantic relations for second string come from Princeton University’s WordNet. Commonness is measured against a corpus of everyday English. Full sources and licences.