Degenerate king leaving. . . (7)
I believe the answer is:
regress
'degenerate' is the definition.
(thesaurus)
'king leaving' is the wordplay.
'king' becomes 'R' (abbreviation for rex, king in Latin).
'leaving' becomes 'egress' (I've seen this before).
'r'+'egress'='REGRESS'
(Other definitions for regress that I've seen before include "Decline" , "Deteriorate" , "Lose ground" , "Get worse or go backwards" , "Revert, throw back" .)