Set free? That's rotten! (6)
I believe the answer is:
putrid
'that's rotten' 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'
(Other definitions for putrid that I've seen before include "Rotting, fetid" , "Really rotten and foul-smelling" , "Decomposing and fetid" , "Decayed, rotten" , "Rancid" .)