Mile or millimeter (4)
I believe the answer is:
unit
'mile or millimeter' is the definition.
(I know that mile is a English unit of measurement)
This is the entire clue.
(Other definitions for unit that I've seen before include "Whole - group" , "Building block - small military force" , "Individual thing as part of a whole" , "Separate part" , "One; military group" .)