He mentions Australian port (6)
I believe the answer is:
sidney
'port' is the definition.
Both the answer and definition are singular nouns.
Perhaps they are linked in a way I don't understand?
'he mentions australian' is the wordplay.
'he' becomes 'Sid'.
'mentions australian' becomes 'ney' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'sid'+'ney'='SIDNEY'
Can you help me to learn more?
(Other definitions for sidney that I've seen before include "Disney over in Australia" , "Sir Philip --, English poet" , "Disney in Oz?" .)