London cops demand for payment (3,4)
I believe the answer is:
the bill
'london' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'cops demand for payment' is the wordplay.
'cops demand' becomes 'the' (I am not sure about this - if you are sure you should believe this answer much more).
'for' means one lot of letters go next to another.
'payment' becomes 'bill'.
'the'+'bill'='THE BILL'
Can you help me to learn more?
(Other definitions for the bill that I've seen before include "requested at end of meal?" , "Soap" .)