Constabulary (6)
I believe the answer is:
police
'constabulary' is the definition.
(I know that constabulary can be written as police)
This is all the clue.
(Other definitions for police that I've seen before include "Limbs of the law" , "... custodians of law and order" , "Branch of the security forces" , "Boys in blue" , "Lawmen" .)