Surplus bowling ball (9)
I believe the answer is:
overspill
'ball' is the definition.
The answer and definition can be both groups as well as being singular nouns.
Perhaps you can see a link between them that I don't see?
'surplus bowling' is the wordplay.
'surplus' becomes 'over' (I've seen this before).
'bowling' becomes 'spill' (I am not sure about this - if you are sure you should believe this answer much more).
'over'+'spill'='OVERSPILL'
Can you help me to learn more?
(Other definitions for overspill that I've seen before include "for people moving out of slums?" , "Surplus population moving elsewhere" , "excessive amount" , "Excess; surplus (population)" , "Excess liquid" .)