Police operation may be a pain (5)
I believe the answer is:
sting
I believe this is a double definition.
'police operation' is the first definition.
(sting can mean a police operation)
'a pain' is the second definition.
(I know that sting is a type of pain)
'may be' acts as a link.
(Other definitions for sting that I've seen before include "Sudden pain - swindle" , "The pain caused by bee or nettle" , "Wound from wasp" , "English musician, b. 1951 -- a sudden pain" , "Bee's weapon" .)