ask for vs excuse

ask for is not a noun while excuse is a noun.

Word	Noun	Adjective	Verb	Adverb
ask for	No	No	Yes	No
excuse	Yes	No	Yes	No

As verbs, excuse is a hyponym of ask for; that is, excuse is a word with a more specific, narrower meaning than ask for:

ask for (noun)	excuse (noun)
	a poor example
	a defense of some offensive behavior or some failure to keep a promise etc.
	a note explaining an absence

ask for (verb)	excuse (verb)
express the need or desire for	accept an excuse for
increase the likelihood of	defend, explain, clear away, or make excuses for by reasoning
	excuse, overlook, or make allowances for; be lenient with
	serve as a reason or cause or justification of
	grant exemption or release to
	ask for permission to be released from an engagement