Free to defend a surprise attack (4)
I believe the answer is:
raid
'surprise attack' is the definition.
(I've seen this before)
'free to defend a' is the wordplay.
'free' becomes 'rid' (to be free of something is to be rid of it).
'to defend' is an insertion indicator.
'rid' enclosing 'a' is 'RAID'.
(Other definitions for raid that I've seen before include "Sudden short attack" , "Inroad" , "Stock Exchange activity" , "Surprise police visit" , "military operation" .)