song vs musical composition

song and musical composition both are nouns.

Word	Noun	Adjective	Verb	Adverb
song	Yes	No	No	No
musical composition	Yes	No	No	No

As nouns, musical composition is a hypernym of song; that is, musical composition is a word with a broader meaning than song:

Other hypernyms of song include composition, opus, piece, piece of music.

song (noun)	musical composition (noun)
the act of singing	a musical work that has been created
a short musical composition with words
the characteristic sound produced by a bird
a distinctive or characteristic sound
a very small sum