I'm great playing jazz (7)
I believe the answer is:
ragtime
'jazz' is the definition.
(thesaurus)
'i'm great playing' is the wordplay.
'playing' indicates an anagram.
'im'+'great'='imgreat'
'imgreat' with letters rearranged gives 'RAGTIME'.
(Other definitions for ragtime that I've seen before include "Scott Joplin's genre" , "Song and dance" , "Maigret (anag)" , "Scott Joplin genre" , "Scott Joplin art form" .)