Gone off a port (5)
I believe the answer is:
genoa
'port' is the definition.
(I've seen this before)
'gone off a' is the wordplay.
'off' is an anagram indicator.
'gone' is an anagram of 'geno'.
'geno'+'a'='GENOA'
(Other definitions for genoa that I've seen before include "Italian city; sail" , "Type of rich fruit cake" , "Seaport in northwestern Italy" , "NW Italian sea port" , "City of Italy" .)