Set free - that's rotten! (6)
I believe the answer is:
'rotten' is the definition.
('putrid' can be a synonym of 'rotten')
'set free' is the wordplay.
'set' becomes 'put' ('put' can be a synonym of 'set').
'free' becomes 'rid' ('rid' can be a synonym of 'free'**).
'that's' is the link.
(Other definitions for putrid that I've seen before include "Rotting, fetid" , "Really rotten and foul-smelling" , "Decomposing and fetid" , "Decayed, rotten" , "Rancid" .)