Law enforcement agency (3)
I believe the answer is:
fbi
'law enforcement agency' is the definition.
(thesaurus)
This is the entire clue.
(Other definitions for fbi that I've seen before include "law enforcers" , "US law enforcement agency" , "J. Edgar Hoover's organisation" , "J. Edgar Hoover long its director" , "Initially, J Edgar Hoover's law force" .)