We initial changes to 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)
'we initial changes' is the wordplay.
'changes' indicates an anagram.
'we'+'initial'='weinitial'
'weinitial' with letters rearranged gives 'LIE IN WAIT'.
'to' acts as a link.
(Other definitions for lie in wait that I've seen before include "hover" , "Hope to catch" , "Plan for ambush" , "Bide time" , "Prepare to attack" .)