Set free, but foul-smelling (6)
I believe the answer is:
putrid
'foul-smelling' is the definition.
(synonyms)
'set free' is the wordplay.
'set' becomes 'put' (synonyms).
'free' becomes 'rid' (to be free of something is to be rid of it).
'put'+'rid'='PUTRID'
'but' is the link.
(Other definitions for putrid that I've seen before include "Decaying, rotting" , "Bad smelling, rotting" , "Rotting, fetid" , "Really rotten and foul-smelling" , "Rancid" .)