In Nine Elms, say, boy drops round sum of money (7)
I believe the answer is:
toponym
'in nine elms say' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'boy drops round sum of money' is the wordplay.
'boy' becomes 'Tom'.
'drops round' indicates putting letters inside.
'sum of money' becomes 'pony' (slang term for 25 pounds).
'tom' going around 'pony' is 'TOPONYM'.
Can you help me to learn more?
(Other definitions for toponym that I've seen before include "Derby, for example" , "Word derived from place name" , "Sevenoaks, say" , "Shetland for instance" , "Geographical word for a place name" .)