Boy with grand air (4)
I believe the answer is:
song
'air' is the definition.
(air can mean a piece of music)
'boy with grand' is the wordplay.
'boy' becomes 'son' (synonyms).
'with' says to put letters next to each other.
'grand' becomes 'g' (G is short for grand as in a thousand).
'son'+'g'='SONG'
(Other definitions for song that I've seen before include "Piece of vocal music" , "Vocal number" , "See 17" , "Vocal composition" , "Melody (which is cheap to go for)" .)