Go back on one's word (6)
I believe the answer is:
renege
'go back on one's word' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for renege that I've seen before include "Go back (on promise etc)" , "Change one's mind" , "Fail to follow suit or fulfil a promise" , "Fail to keep promise, or revoke in cards" , "Deny or renounce" .)