Go back on a promise (6)
I believe the answer is:
'go back on a promise' 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)" , "Deny or renounce" , "Fail to fulfil a promise or obligation" , "Fail to follow suit at cards" , "In card games, revoke" .)