Clue: One landing fish in port wandering round Exeter once (8)
Answer: PISCATOR
Explanation: Definition: One landing fish
ISCA (Roman name for what is now Exeter, in full Isca Dumnoniorum) in PORT; ISCA and EXE are related names for the river, from an Celtic word for fish (also seen in Scottish Gaelic "iasg" and Irish "iasc")