What poor have that rich want for? (7)
I believe the answer is:
nothing
'have that rich want for?' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'what poor' is the wordplay.
I cannot really understand how this works, but
'poor' could be 'thin' (I've seen this in other clues) and 'thin' is present in the answer.
The remaining letters 'nog' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
(Other definitions for nothing that I've seen before include "can mean love" , "'Zero, zilch (7)'" , "what House of Peers did particularly well?" , "What's in a vacuum?" , "Zilch, nada" .)