Fail to keep a promise (6)
I believe the answer is:
renege
'fail to keep a promise' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for renege that I've seen before include "Withdraw" , "In card games, revoke" , "Fail to follow suit or fulfil a promise" , "South American cowboy" , "Fail to carry out a commitment" .)