Police operation is a pain (5)
I believe the answer is:
sting
This clue is a double definition.
'police operation' is the first definition.
(sting can mean a police operation)
'pain' is the second definition.
(sting is a kind 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" .)