No prohibition for port (4)
I believe the answer is:
oban
'for port' is the definition.
(Oban is a Scottish port)
'no prohibition' is the wordplay.
'no' becomes 'O' (resembles 0 - 'no' can mean 'zero').
'prohibition' becomes 'ban' (ban is a kind of prohibition).
'o'+'ban'='OBAN'
(Other definitions for oban that I've seen before include "Argyll and Bute resort" , "Scottish town" , "Port in W. Scotland" , "Port in western Scotland" , "Scottish west coast resort" .)