Foul-smelling old hat (4)
I believe the answer is:
olid
'foul-smelling' is the definition.
(I've seen this before)
'old hat' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'hat' becomes 'lid' (synonyms).
'o'+'lid'='OLID'
(Other definitions for olid that I've seen before include "Rank-smelling" , "rank" , "evil-smelling" , "Foul-smelling" .)