Mariner at the end seen in worst French port (5)
I believe the answer is:
brest
'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 "French port" , "Port in Brittany" , "Naval base in Brittany" , "French warships here in Brittany" , "French Atlantic naval base" .)