Rainfall measure (4)
I believe the answer is:
inch
'rainfall measure' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for inch that I've seen before include "Edge (forwards)" , "Move in tiny increments" , "Move slowly, in small steps" , "Subdivision of 14 down" , "One-twelfth of a foot" .)