I'm great, playing old American music (7)
I believe the answer is:
ragtime
'old american music' is the definition.
'i'm great playing' is the wordplay.
'playing' is an anagram indicator.
'im'+'great'='imgreat'
'imgreat' is an anagram of 'RAGTIME'.
(Other definitions for ragtime that I've seen before include "Kind of music" , "Form of jazz with a two-four rhythm" , "Migrate to find jazz music" , "Type of music" , "Song and dance" .)