I'm great playing music (7)
I believe the answer is:
'music' is the definition.
(ragtime is a kind of music)
'i'm great playing' is the wordplay.
'playing' is an anagram indicator.
'imgreat' with letters rearranged gives 'RAGTIME'.
(Other definitions for ragtime that I've seen before include "Syncopated piano music - migrate (anag)", "Style of jazz popularised by Scott Joplin et al", "Scott Joplin art form", "Kind of jazz for Maigret", "Type of music".)