Be unfaithful to (3-4)
I believe the answer is:
two-time
'be unfaithful to' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for two-time that I've seen before include "Be unfaithful to a lover" , "Be unfaithful to (a partner)" , "Deceive slangily - twice?" , "take her for a ride" , "Play away?" .)