understudy vs player

understudy and player both are nouns.

Word	Noun	Adjective	Verb	Adverb
understudy	Yes	No	Yes	No
player	Yes	No	No	No

As nouns, player is a hypernym of understudy; that is, player is a word with a broader meaning than understudy:

Other hypernyms of understudy include actor, histrion, role player, thespian.

understudy (noun)	player (noun)
an actor able to replace a regular performer when required	a theatrical performer
	a person who participates in or is skilled at some game
	someone who plays a music as a profession
	a person who pursues a number of different social and sexual partners simultaneously
	someone who takes part in an activity

understudy (verb)	player (verb)
be an understudy or alternate for a role