Musical with hard song (4)
I believe the answer is:
hair
'musical' is the definition.
(I've seen this before)
'with hard song' is the wordplay.
'with' means one lot of letters go next to another.
'hard' becomes 'h'.
'song' becomes 'air' (I've seen this before).
'h'+'air' is 'HAIR'.
(Other definitions for hair that I've seen before include "Famous hippy musical of hirsute character" , "Growth from skin" , "Barnet" , "Coiffure" , "Whiskers" .)