Prepare an ambush (3,2,4)
I believe the answer is:
lie in wait
'prepare an ambush' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for lie in wait that I've seen before include "Plan to attack" , "Be prepared to ambush" , "Plan for ambush" , "Set an ambush" , "Be ready to surprise" .)