flattop vs combat ship

flattop and combat ship both are nouns.

Word	Noun	Adjective	Verb	Adverb
flattop	Yes	No	No	No
combat ship	Yes	No	No	No

As nouns, combat ship is a hypernym of flattop; that is, combat ship is a word with a broader meaning than flattop:

flattop: a large warship that carries planes and has a long flat deck for takeoffs and landings
combat ship: a government ship that is available for waging war

Other hypernyms of flattop include war vessel, warship.

flattop (noun)	combat ship (noun)
a large warship that carries planes and has a long flat deck for takeoffs and landings	a government ship that is available for waging war
a closely cropped haircut; usually for men or boys