Narc's measure (4)
I believe the answer is:
kilo
'narc's measure' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for kilo that I've seen before include "Metric unit of weight" , "Unit of mass" , "Unit of weight (abbr)" , "possible drugs unit" , "Metric prefix meaning 1,000" .)