Free to cover a sudden attack (4)
I believe the answer is:
raid
'sudden attack' is the definition.
(I've seen this before)
'free to cover a' is the wordplay.
'free' becomes 'rid' ('rid' can be a synonym of 'free'**).
'to cover' indicates putting letters inside.
'rid' enclosing 'a' is 'RAID'.
(Other definitions for raid that I've seen before include "Invade" , "Search by police without warning" , "Foray, sudden attack" , "Maraud" , "Finch and Bronson starred in . . . . On Entebbe" .)