Sustained campaign about European clothing (4)
I believe the answer is:
wear
'clothing' is the definition.
('wear' can be a synonym of 'clothing')
'sustained campaign about european' is the wordplay.
'sustained campaign' becomes 'war'.
'about' indicates putting letters inside.
'european' becomes 'e' (abbreviation e.g. EU).
'war' placed around 'e' is 'WEAR'.
(Other definitions for wear that I've seen before include "Apparel" , "Northern runner" , "Damage from use" , "Have on - deteriorate from constant use" , "Have on; fray" .)