Not so foul proposition - not so wrong (7)
I believe the answer is:
illicit
'wrong' is the definition.
(I've seen this before)
'not so foul proposition not' is the wordplay.
I cannot quite see how this works, but
'foul' could be 'ill' and 'ill' is present in the answer.
This explanation may well be incorrect...
'so' acts as a link.
Can you help me to learn more?
(Other definitions for illicit that I've seen before include "Contrary to or forbidden by law" , "Illegal" , "wrong" , "Without approval" , "Not allowed, unlawful" .)