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' is a reversal indicator.
'egener' is hidden within 'degenerate'.
'egener' written backwards gives '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" .)