I'm great playing music (7)
I believe the answer is:
ragtime
'music' is the definition.
(ragtime is a kind of music)
'i'm great playing' is the wordplay.
'playing' is an anagram indicator.
'im'+'great'='imgreat'
'imgreat' with letters rearranged gives 'RAGTIME'.
(Other definitions for ragtime that I've seen before include "musical style" , "Kind of jazz for Maigret" , "Style of piano music" , "Jazz music for Maigret" , "Syncopated jazz style" .)