Set free, is really bad (6)

I believe the answer is:

putrid

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'really bad' is the definition.
'putrid' can be an answer for 'bad' (thesaurus). I am unsure of the 'really' bit.

'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'

'is' is the link.

Can you help me to learn more?

(Other definitions for putrid that I've seen before include "Rotting and foul-smelling" , "contaminated" , "Bad smelling, rotting" , "Foul" , "Decomposed, rotting" .)

I've seen this clue in the Evening Standard.

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