Beer wasted in population leaving London? (9)
I believe the answer is:
overspill
'beer' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for overspill that I've seen before include "Surplus population moving elsewhere" , "Excess; surplus (population)" , "for people moving out of slums?" , "Excess liquid" , "excessive amount" .)