The man's inside playing subtle form of music (3,5)
I believe the answer is:
the blues
'form of music' is the definition.
'the man's inside playing subtle' is the wordplay.
'the man' becomes 'he'.
'inside' is an insertion indicator.
'playing' indicates anagramming the letters.
'subtle' anagrammed gives 'tblues'.
'he' put into 'tblues' is 'THE BLUES'.
(Other definitions for the blues that I've seen before include "Old British regiment that fought up" , "Bout of depression" , "Royal Navy may exemplify?" , "Lady Sings - -, Billie Holliday autobiography" , "E.g. Manchester City" .)