Free to include a surprise attack (4)
I believe the answer is:
raid
'surprise attack' is the definition.
(I've seen this before)
'free to include a' is the wordplay.
'free' becomes 'rid' (to be free of something is to be rid of it).
'to include' is an insertion indicator.
'rid' going around 'a' is 'RAID'.
(Other definitions for raid that I've seen before include "Foray, sudden attack" , "Assault, attack" , "Inroad" , "Surprise police visit" , "bust" .)