Old boy and knight drinking a port (4)
I believe the answer is:
oban
'port' is the definition.
(I've seen this before)
'old boy and knight drinking a' is the wordplay.
'old boy' becomes 'ob'.
'and' says to put letters next to each other.
'knight' becomes 'n' (chess).
'drinking' means one lot of letters goes inside another.
'ob'+'n'='obn'
'obn' placed around 'a' is 'OBAN'.
(Other definitions for oban that I've seen before include "Port in western Scotland" , "Port and holiday centre on the west coast of Scotland" , "Argyll seaport" , "Scottish port and resort" , "Scottish west coast resort" .)