singer vs torch singer

singer and torch singer both are nouns.

Word	Noun	Adjective	Verb	Adverb
singer	Yes	No	No	No
torch singer	Yes	No	No	No

As nouns, torch singer is a hyponym of singer; that is, torch singer is a word with a more specific, narrower meaning than singer:

singer: a person who sings
torch singer: a singer (usually a woman) who specializes in singing torch songs

singer (noun)	torch singer (noun)
a person who sings	a singer (usually a woman) who specializes in singing torch songs