Old port can be a drink knocked back at noon (5)
I believe the answer is:
pusan
'old port' is the definition.
(former spelling for the South Korean port Busan)
'a drink knocked back at noon' is the wordplay.
'drink' becomes 'sup' (I've seen this before**).
'knocked back' says the letters should be written in reverse.
'at' means one lot of letters go next to another.
'noon' becomes 'n'.
'a'+'sup'='asup'
'asup' back-to-front is 'pusa'.
'pusa'+'n'='PUSAN'
'can be' acts as a link.
(Another definition for pusan that I've seen is " Largest port in South Korea".)