Set free, having gone off (6)
I believe the answer is:
putrid
'off' is the definition.
(I've seen this before)
'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'
'having gone' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for putrid that I've seen before include "Really rotten and foul-smelling" , "Rotting and foul-smelling" , "Turned" , "contaminated" , "Rotting, fetid" .)