River rapid on fresh bit of water (8)
I believe the answer is:
raindrop
'bit of water' is the definition.
'river rapid on fresh' is the wordplay.
'river' becomes 'r'.
'fresh' indicates anagramming the letters.
'rapid'+'on'='rapidon'
'rapidon' with letters rearranged gives 'aindrop'.
'r'+'aindrop'='RAINDROP'
(Other definitions for raindrop that I've seen before include "Precipitated globule - keeps falling on my head in song" , "Pro-drain (anag)" , "tiny amount of water" , "Bead of water from a cloud" , "One of many in a shower" .)