American silly wearing sleeveless jacket (6)
I believe the answer is:
jerkin
'sleeveless jacket' is the definition.
(I've seen this before)
'american silly wearing' is the wordplay.
'american silly' becomes 'jerk' (US term for a silly person).
'wearing' becomes 'in' (wear in (and 'in' is fashionable)).
'jerk'+'in'='JERKIN'
(Other definitions for jerkin that I've seen before include "Type of coat or jacket" , "Close-fitting sleeveless jacket" , "Leatherjacket" , "sleeveless garment" , "jacket with no sleeves or collar" .)