Don't bet on it (3)
I believe the answer is:
nag
'don't bet on it' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for nag that I've seen before include "Remind constantly" , "Henpeck, scold non-stop" , "Scold persistently, annoy" , "to get on one's back" , "Poor horse" .)