Police operation is a pain (5)
I believe the answer is:
sting
I believe this is a double definition.
'police operation' is the first definition.
(I've seen this in another clue)
'pain' is the second definition.
(I know that sting is a type of pain)
'is a' acts as a link.
(Other definitions for sting that I've seen before include "con trick" , "Smart - cheat" , "18, sharp pain" , "Bee/wasp wound" , "Tingle sharply" .)