We initial changes to prepare ambush (3,2,4)
I believe the answer is:
lie in wait
'prepare ambush' is the definition.
(as in lying in wait ready to ambush someone)
'we initial changes' is the wordplay.
'changes' is an anagram indicator.
'we'+'initial'='weinitial'
'weinitial' is an anagram of 'LIE IN WAIT'.
'to' acts as a link.
(Other definitions for lie in wait that I've seen before include "prepare a trap" , "Prepare to attack" , "what will hunter do?" , "Be prepared to ambush" , "Lurk" .)