Five hundred pounds, but apparently not a thousand? (5)
I believe the answer is:
money
'five' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'hundred pounds but apparently not a thousand?' is the wordplay.
I cannot really understand how this works, but
'hundred' could be 'one' (synonyms) and 'one' is located in the answer.
'thousand?' could be 'm' ('m' can be a synonym of 'thousand') and 'm' is located in the answer.
A single letter 'y' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for money that I've seen before include "ready" , "See 7" , "Legal tender" , "Mazuma, ackers or dosh" , "Lucre" .)