Degenerate holding back default (6)
I believe the answer is:
renege
'default' is the definition.
(I've seen this before)
'degenerate holding back' is the wordplay.
'holding' says the answer is hidden in the clue.
'back' shows that the letters should be reversed in order.
'egener' is hidden in the letters of 'degenerate'.
'egener' in reverse letter order is 'RENEGE'.
(Other definitions for renege that I've seen before include "Don't follow suit when you can" , "Default" , "Deny or renounce" , "Go back on one's word, break promise" , "Fail to keep promise, or revoke in cards" .)