Prohibition affecting nobleman (5)
I believe the answer is:
baron
'nobleman' is the definition.
(baron is a kind of nobleman)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for baron that I've seen before include "Noble rank" , "captain of industry" , "Lowest rank of British peerage" , "Title of rank" , "Minor nobleman" .)