trews vs pair of trousers

trews and pair of trousers both are nouns.

Word	Noun	Adjective	Verb	Adverb
trews	Yes	No	No	No
pair of trousers	Yes	No	No	No

As nouns, pair of trousers is a hypernym of trews; that is, pair of trousers is a word with a broader meaning than trews:

trews: (used in the plural) tight-fitting trousers; usually of tartan
pair of trousers: (usually in the plural) a garment extending from the waist to the knee or ankle, covering each leg separately

Other hypernyms of trews include pant, trousers.

trews (noun)	pair of trousers (noun)
(used in the plural) tight-fitting trousers; usually of tartan	(usually in the plural) a garment extending from the waist to the knee or ankle, covering each leg separately