Criminal is a rotten closer (8)
I believe the answer is:
offender
'criminal' is the definition.
(I know that criminal can be written as offender)
'rotten closer' is the wordplay.
'rotten' becomes 'off' (rotten food is 'off').
'closer' becomes 'ender' (I've seen this in another clue).
'off'+'ender'='OFFENDER'
'is a' acts as a link.
(Other definitions for offender that I've seen before include "Wrongdoer" , "miscreant" , "Delinquent" , "One breaking rule" , "Committer of illegal act" .)