Prepare an ambush (3,2,4)
I believe the answer is:
lie in wait
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">prepare an ambush</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)