A little water moving penny down drain, runs over penny (8)
I believe the answer is:
raindrop
'a little water' is the definition.
'moving penny down drain runs over penny' is the wordplay.
'moving penny down' becomes 'r' (I am not sure about this - if you are sure you should believe this answer much more).
'runs' indicates an anagram.
'over' becomes 'o' (cricket abbreviation).
'penny' becomes 'p' (short for pence).
'drain' is an anagram of 'aindr'.
'r'+'aindr'+'o'+'p'='RAINDROP'
Can you help me to learn more?
(Other definitions for raindrop that I've seen before include "Having a bit of a fall" , "Bead of water from a cloud" , "Part of a shower" , "Spot of water; a Chopin prelude" , "Precipitated globule - keeps falling on my head in song" .)