Detachment for security or reconnaissance (6)
I believe the answer is:
patrol
'detachment for security or reconnaissance' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for patrol that I've seen before include "Reconnaissance unit" , "Scouting expedition" , "Small group of Guides/Scouts" , "military unit" , "Move round and keep watch" .)