Left or right in vessel (4)
I believe the answer is:
port
'left' is the definition.
(left side of a ship - opposite of starboard)
'right in vessel' is the wordplay.
'right' becomes 'r' (common abbreviation).
'in' means one lot of letters goes inside another.
'vessel' becomes 'pot' (pot is a kind of vessel**).
'r' put within 'pot' is 'PORT'.
'or' is the link.
(Other definitions for port that I've seen before include "Drink passed to the left" , "Popular after dinner liqueur" , "After-dinner drink" , "Drink - left at sea" , "'Any . . . . In a storm'" .)