a capella singing vs singing

a capella singing and singing both are nouns.

Word	Noun	Adjective	Verb	Adverb
a capella singing	Yes	No	No	No
singing	Yes	Yes	No	No

As nouns, singing is a hypernym of a capella singing; that is, singing is a word with a broader meaning than a capella singing:

Other hypernyms of a capella singing include vocalizing.

a capella singing (noun)	singing (noun)
singing without instrumental accompaniment	the act of singing vocal music
	disclosing information or giving evidence about another

a capella singing (adjective)	singing (adjective)
	smooth and flowing