Rainfall measurement unit (4)
I believe the answer is:
inch
'rainfall measurement unit' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for inch that I've seen before include "One twelfth of a foot" , "25.4 millimetres" , "Move slowly and cautiously" , "One point 63380 th of a mile" , "See 12" .)