A bit of precipitation and prior storm (8)
I believe the answer is:
raindrop
'a bit of precipitation' is the definition.
(I've seen this before)
'and prior storm' is the wordplay.
'storm' indicates an anagram.
'and'+'prior'='andprior'
'andprior' is an anagram of 'RAINDROP'.
(Other definitions for raindrop that I've seen before include "Spot of precipitation" , "Precipitated globule - keeps falling on my head in song" , "bit of the wet stuff" , "Part of a shower" , "Unit of moisture precipitated from sky" .)