Bit of precipitation (8)
I believe the answer is:
raindrop
'bit of precipitation' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for raindrop that I've seen before include "Water from the heavens" , "Globule of precipitated moisture" , "Little evidence of wet weather" , "Part of a shower" , "Pro-drain (anag)" .)