Be prone to ambush (3,2,4)
I believe the answer is:
lie in wait
'ambush' is the definition.
(I've seen this before)
'be prone' is the wordplay.
'be' becomes 'lie in' (lying in is a kind of being).
'prone' becomes 'wait' (I can't explain this - if you can you should give a lot more credence to this answer).
'liein'+'wait'='LIE IN WAIT'
'to' is the link.
Can you help me to learn more?
(Other definitions for lie in wait that I've seen before include "as some bushwhackers do" , "Plan to attack" , "prepare a trap" , "Prepare to attack" , "Lurk" .)