Degenerate's expression of contentment, lying back in bed (7)
I believe the answer is:
corrupt
'degenerate's' is the definition.
(thesaurus)
'expression of contentment lying back in bed' is the wordplay.
'expression of contentment' becomes 'purr' (I've seen this in another clue).
'lying back' is a reversal indicator.
'in' indicates putting letters inside.
'bed' becomes 'cot' (cot is a type of bed).
'purr' written backwards gives 'rrup'.
'rrup' inserted within 'cot' is 'CORRUPT'.
(Other definitions for corrupt that I've seen before include "spoilt" , "Taint" , "rot" , "Willing to act dishonestly in return for personal gain" , "Criminal" .)