Small animal dropping (4)
I believe the answer is:
scat
'dropping' is the definition.
(I've seen this before)
'small animal' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'animal' becomes 'cat' (cats are animals).
's'+'cat'='SCAT'
(Other definitions for scat that I've seen before include "Bolt" , "Jazz singing - animal droppings - silvery fish" , "vocal style?" , "Type of jazz singing - be off!" , "Buzz off" .)