A vessel is in port (5)
I believe the answer is:
tunis
'port' is the definition.
(I know that Tunis is a port)
'a vessel is' is the wordplay.
'a vessel' becomes 'tun' (type of vessel for beer).
'tun'+'is'='TUNIS'
'in' acts as a link.
(Other definitions for tunis that I've seen before include "North African city" , "Mediterranean capital and port" , "North African port and capital" , "I stun (anag) - port and capital" , "North African location" .)