Don't do this. Keep your promise and you'll secure a pact from a percentage of them at least. (6)
I believe the answer is:
renege
'don't do this keep your promise and you'll secure a pact from a percentage of them at least' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
This is the entire clue.
Can you help me to learn more?
(Other definitions for renege that I've seen before include "Go back on one's word, break promise" , "South American cowboy" , "Fail to follow suit at cards" , "In card games, revoke" , "Go back (on promise etc)" .)