singing vs scat singing

singing and scat singing both are nouns.

Word	Noun	Adjective	Verb	Adverb
singing	Yes	Yes	No	No
scat singing	Yes	No	No	No

As nouns, scat singing is a hyponym of singing; that is, scat singing is a word with a more specific, narrower meaning than singing:

singing: the act of singing vocal music
scat singing: singing jazz; the singer substitutes nonsense syllables for the words of the song and tries to sound like a musical instrument

singing (noun)	scat singing (noun)
the act of singing vocal music	singing jazz; the singer substitutes nonsense syllables for the words of the song and tries to sound like a musical instrument
disclosing information or giving evidence about another

singing (adjective)	scat singing (adjective)
smooth and flowing