Some policemen are pests (4)
I believe the answer is:
lice
'pests' is the definition.
(I've seen this in another clue)
'some policemen are' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'LICE' is hidden within 'policemen are'.
(Other definitions for lice that I've seen before include "Crawlers" , "Bloodsucking parasitic insect" , "Bloodsucking insects" , "Parasitic insects" , "Wingless bloodsucking parasites" .)