Old Greek article on prohibition (6)
I believe the answer is:
theban
'old greek' is the definition.
'theban' can be an answer for 'greek' (I've seen this before). I am not sure about the 'old' bit.
'article on prohibition' is the wordplay.
'article' becomes 'the' (the 'definite article' part of speech).
'on' is a charade indicator (letters next to each other).
'prohibition' becomes 'ban' (ban is a kind of prohibition).
'the'+'ban'='THEBAN'
Can you help me to learn more?
(Other definitions for theban that I've seen before include "Old citizen" , "ancient Greek citizen" , "Old Egyptian" , "Inhabitant of an ancient Egyptian capital" , "Nab the person from the ancient Greek city" .)