Idiot wearing sleeveless jacket (6)
I believe the answer is:
jerkin
'sleeveless jacket' is the definition.
(historical type of garment)
'idiot wearing' is the wordplay.
'idiot' becomes 'jerk' (I've seen this in another clue).
'wearing' becomes 'in' (as in being 'in' certain clothes).
'jerk'+'in'='JERKIN'
(Other definitions for jerkin that I've seen before include "jacket with no sleeves or collar" , "Type of coat or jacket" , "sort of jacket" , "Close-fitting sleeveless jacket" , "sleeveless garment" .)