Regular tour to maintain security (6)
I believe the answer is:
patrol
'regular tour to maintain security' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for patrol that I've seen before include "military unit" , "Portal for those on security duty" , "Detachment for security or reconnaissance" , "Scouting expedition" , "Part of Scouts troop" .)