We, with initial manoeuvring, remain hidden while preparing to attack (3,2,4)
I believe the answer is:
lie in wait
'remain hidden while preparing to attack' is the definition.
(I've seen this before)
'we with initial manoeuvring' is the wordplay.
'with' is a charade indicator (letters next to each other).
'manoeuvring' indicates anagramming the letters.
'we'+'initial'='weinitial'
'weinitial' with letters rearranged gives 'LIE IN WAIT'.
(Other definitions for lie in wait that I've seen before include "Prepare to launch ambush" , "Plan for ambush" , "Bide time" , "Be prepared to ambush" , "as some bushwhackers do" .)