Throw small ball (5)
I believe the answer is:
spill
'throw small ball' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Maybe there's a link between them I don't understand?
'small ball' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'ball' becomes 'pill' (humorous term for the ball in some sports).
's'+'pill'='SPILL'
Can you help me to learn more?
(Other definitions for spill that I've seen before include "Tip" , "Waste (liquid)" , "Upset; brim over" , "Cause to fall from horse" , "Thin piece of wood or rolled paper to light a fire" .)