Smart to follow each part of grid reference (7)
I believe the answer is:
easting
'part of grid reference' is the definition.
(compass point)
'smart to follow each' is the wordplay.
'smart' becomes 'sting' (both can mean to burn or hurt).
'to follow' is a charade indicator (letters next to each other) (some letters go after or follow others).
'each' becomes 'ea' (abbreviation).
'sting' put after 'ea' is 'EASTING'.
(Other definitions for easting that I've seen before include "distance for navigator" , "Map reference" , "distance measured" , "oriental course?" , "distance in another direction" .)