slack suit vs suit of clothes

slack suit and suit of clothes both are nouns.

Word	Noun	Adjective	Verb	Adverb
slack suit	Yes	No	No	No
suit of clothes	Yes	No	No	No

As nouns, suit of clothes is a hypernym of slack suit; that is, suit of clothes is a word with a broader meaning than slack suit:

slack suit: casual dress consisting of slacks and matching jacket
suit of clothes: a set of garments (usually including a jacket and trousers or skirt) for outerwear all of the same fabric and color

Other hypernyms of slack suit include suit.

slack suit (noun)	suit of clothes (noun)
casual dress consisting of slacks and matching jacket	a set of garments (usually including a jacket and trousers or skirt) for outerwear all of the same fabric and color