Break a promise (6)
I believe the answer is:
renege
'break 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 "Don't follow suit when you can" , "Go back on an obligation" , "Change one's mind" , "Fail to honour an obligation" , "Fail to follow suit at cards" .)