pinch hitter vs second-stringer

pinch hitter and second-stringer both are nouns.

Word	Noun	Adjective	Verb	Adverb
pinch hitter	Yes	No	No	No
second-stringer	Yes	No	No	No

As nouns, second-stringer is a hypernym of pinch hitter; that is, second-stringer is a word with a broader meaning than pinch hitter:

pinch hitter: (baseball) a substitute for the regular batter
second-stringer: an athlete who plays only when a starter on the team is replaced

Other hypernyms of pinch hitter include batsman, batter, hitter, slugger, reserve, substitute.

pinch hitter (noun)	second-stringer (noun)
(baseball) a substitute for the regular batter	an athlete who plays only when a starter on the team is replaced