Useless person wearing sleeveless jacket (6)
I believe the answer is:
jerkin
'sleeveless jacket' is the definition.
(historical type of garment)
'useless person wearing' is the wordplay.
'useless person' becomes 'jerk' (term for a stupid or inept person).
'wearing' becomes 'in' (as in being 'in' certain clothes).
'jerk'+'in'='JERKIN'
(Other definitions for jerkin that I've seen before include "Short coat" , "Tight, sleeveless, collarless jacket" , "jacket with no sleeves or collar" , "Sleveless jacket" , "sort of jacket" .)