Mariner at the end seen in worst French port (5)
I believe the answer is:
Here is my best explanation:
'french port' is the definition.
(a port city in brittany)
'mariner at the end seen in worst' is the wordplay.
'at the end' indicates one should take the final letters.
'seen in' means one lot of letters goes inside another.
'worst' becomes 'best' (both can mean to beat someone).
The final letter of 'mariner' is 'r'.
'r' going into 'best' is 'BREST'.
(Other definitions for brest that I've seen before include "Brittany port", "French port", "Naval base in Brittany", "French warships here in Brittany", "Port in Brittany", "French Atlantic naval base" and "Breton port".)