Pot fish? William in casting place? (7,5)
I believe the answer is:
polling booth
'casting place?' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'pot fish? william' is the wordplay.
'pot' becomes 'Pol' (Pol Pot).
'fish?' becomes 'ling' (ling is a kind of fish).
'william' becomes 'booth' (I can't explain this - if you can you should give a lot more credence to this answer).
'pol'+'ling'+'booth'='POLLING-BOOTH'
'in' acts as a link.
Can you help me to learn more?
(Another definition for polling booth that I've seen is " Place to register vote".)