Idiot wearing jacket with sleeves missing (6)
I believe the answer is:
jerkin
'jacket with sleeves missing' is the definition.
'idiot wearing' is the wordplay.
'idiot' becomes 'jerk'.
'wearing' becomes 'in' (wear in (and 'in' is fashionable)).
'jerk'+'in'='JERKIN'
(Other definitions for jerkin that I've seen before include "leather jacket" , "jacket with no sleeves or collar" , "sort of jacket" , "Sleveless jacket" , "Tight, sleeveless, collarless jacket" .)