Air therefore no good (4)
I believe the answer is:
song
'air' is the definition.
(air can mean a piece of music)
'therefore no good' is the wordplay.
'therefore' becomes 'so' ('so' can mean 'consequently' or 'therefore').
'no good' becomes 'ng' (abbreviation).
'so'+'ng'='SONG'
(Other definitions for song that I've seen before include "Piece of music, usually with words" , "Melody (which is cheap to go for)" , "Vocal number" , "You work best when on this" , "See 17" .)