second fiddle vs underling

second fiddle and underling both are nouns.

Word	Noun	Adjective	Verb	Adverb
second fiddle	Yes	No	No	No
underling	Yes	No	No	No

As nouns, underling is a hypernym of second fiddle; that is, underling is a word with a broader meaning than second fiddle:

second fiddle: someone who serves in a subordinate capacity or plays a secondary role
underling: an assistant subject to the authority or control of another

Other hypernyms of second fiddle include foot soldier, subordinate, subsidiary.

second fiddle (noun)	underling (noun)
a secondary role or function	an assistant subject to the authority or control of another
someone who serves in a subordinate capacity or plays a secondary role