One French port is a key to the others (5)
I believe the answer is:
brest
'one french port' is the definition.
(a port city in brittany)
'key to the others' is the wordplay.
'key' becomes 'b' (eg B major, musical key**).
'to the others' becomes 'rest' (I've seen this in other clues).
'b'+'rest'='BREST'
'is a' is the link.
(Other definitions for brest that I've seen before include "French port" , "Breton port" , "French warships here in Brittany" , "Port in Brittany" , "Naval base in Brittany" .)