Vessel brought up crude oil in port (6)
I believe the answer is:
napoli
'port' is the definition.
(I know that Napoli is a port)
'vessel brought up crude oil' is the wordplay.
'vessel' becomes 'pan' (vessel or container for cooking).
'brought up' is a reversal indicator (in a down clue, letters go up).
'crude' is an anagram indicator.
'pan' in reverse letter order is 'nap'.
'oil' with letters rearranged gives 'oli'.
'nap'+'oli'='NAPOLI'
'in' is the link.
(Other definitions for napoli that I've seen before include "Decline" , "Italian name for Naples" , "Mediterranean city" , "Italian city" .)