Rain very lightly in small mine (4)
I believe the answer is:
spit
'rain very lightly' is the definition.
(I've seen this before)
'small mine' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'mine' becomes 'pit' (down the pit).
's'+'pit'='SPIT'
'in' is the link.
(Other definitions for spit that I've seen before include "cook a roast?" , "Expectorate into pits" , "Show disdain" , "issue with some violence" , "Coastal feature, tongue of sand" .)