Grain measure (6)
I believe the answer is:
bushel
'grain measure' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for bushel that I've seen before include "Dry measure of eight gallons" , "Unit of dry or liquid measure" , "'Don't hide your light behind a . . . . . .'" , "Eight-gallon measure of capacity - under which one's light should not be hidden" , "Four pecks" .)