second-rater vs doer

second-rater and doer both are nouns.

Word	Noun	Adjective	Verb	Adverb
second-rater	Yes	No	No	No
doer	Yes	No	No	No

As nouns, doer is a hypernym of second-rater; that is, doer is a word with a broader meaning than second-rater:

Other hypernyms of second-rater include actor, worker.

second-rater (noun)	doer (noun)
a person of second-rate ability or value	a person who acts and gets things done