Ragtime composition for Simenon's detective (7)
'simenon's detective' is the definition.
(I've seen this before)
'ragtime composition' is the wordplay.
'composition' indicates anagramming the letters.
'ragtime' with letters rearranged gives 'MAIGRET'.
'for' is the link.
(Other definitions for maigret that I've seen before include "Policeman", "cracker?", "Simenon''s detective-superintendent", "Simenon's Inspector", "Fictional detective".)