roast lamb vs roast

roast lamb and roast both are nouns.

Word	Noun	Adjective	Verb	Adverb
roast lamb	Yes	No	No	No
roast	Yes	Yes	Yes	No

As nouns, roast is a hypernym of roast lamb; that is, roast is a word with a broader meaning than roast lamb:

roast lamb: a cut of lamb suitable for roasting
roast: a piece of meat roasted or for roasting and of a size for slicing into more than one portion

Other hypernyms of roast lamb include joint.

roast lamb (noun)	roast (noun)
a cut of lamb suitable for roasting	negative criticism
	a piece of meat roasted or for roasting and of a size for slicing into more than one portion

roast lamb (adjective)	roast (adjective)
	(meat) cooked by dry heat in an oven

roast lamb (verb)	roast (verb)
	cook with dry heat, usually in an oven
	subject to laughter or ridicule