Port has only swimmer (5,4)
I believe the answer is:
dover sole
'swimmer' is the definition.
(type of fish)
'port has only' is the wordplay.
'port' becomes 'dover' (English port).
'has' is a charade indicator (letters next to each other).
'only' becomes 'sole' (synonyms).
'dover'+'sole'='DOVER SOLE'
(Other definitions for dover sole that I've seen before include "Catch" , "Kentish flat fish" .)