Rotten sound of cat getting back into bed (7)
I believe the answer is:
corrupt
'rotten' is the definition.
(synonyms)
'sound of cat getting back into bed' is the wordplay.
'sound of cat' becomes 'purr' (I've seen this before).
'getting back' shows that the letters should be reversed in order.
'into' is an insertion indicator.
'bed' becomes 'cot' (cot is a kind of bed).
'purr' in reverse letter order is 'rrup'.
'rrup' put within 'cot' is 'CORRUPT'.
(Other definitions for corrupt that I've seen before include "doctor" , "Crooked" , "Rotten; bribable" , "Criminal" , "Evil or morally depraved" .)