What degenerate keeps back to break an agreement (6)
I believe the answer is:
renege
'break an agreement' is the definition.
(as in reneging on a promise)
'what degenerate keeps back' is the wordplay.
'keeps' indicates a hidden word.
'back' says the letters should be written backwards.
'egener' can be found hidden inside 'what degenerate'.
'egener' back-to-front is 'RENEGE'.
'to' is the link.
(Other definitions for renege that I've seen before include "Fail to honour an obligation" , "South American cowboy" , "Withdraw" , "Fail to follow suit or fulfil a promise" , "Fail to fulfil a promise or obligation" .)