Be untruthful with (3,2)
I believe the answer is:
lie to
'be untruthful with' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for lie to that I've seen before include "Remain motionless" , "See 10 Across" , "Become stationary (naut.)" , "Misinform" , "ship's to show no sign of movement" .)