Starter of oysters with effervescent Asti and old port (5)
I believe the answer is:
ostia
'old port' is the definition.
(I've seen this before)
'starter of oysters with effervescent asti' is the wordplay.
'starter of' says to take the initial letters.
'with' means one lot of letters go next to another.
'effervescent' indicates an anagram.
The first letter of 'oysters' is 'o'.
'asti' anagrammed gives 'stia'.
'o'+'stia'='OSTIA'
'and' is the link.
(Other definitions for ostia that I've seen before include "Roman port" , "town near Rome" , "Small openings" , "The old port of Rome" , "ancient town" .)