Criminal rip-off in fashion (7)
I believe the answer is:
culprit
'criminal' is the definition.
(I've seen this before)
'rip-off in fashion' is the wordplay.
'off' is an anagram indicator.
'in' means one lot of letters goes inside another.
'fashion' becomes 'cult' (cult is a kind of fashion).
'rip' anagrammed gives 'pri'.
'pri' put within 'cult' is 'CULPRIT'.
(Other definitions for culprit that I've seen before include "one awaiting trial" , "Offender or guilty party" , "he should have conviction" , "One perpetrating crime" , "The person to blame, the guilty one" .)