The French port is very heavy (6)
I believe the answer is:
leaden
'very heavy' is the definition.
'the french port' is the wordplay.
'the french' becomes 'le'.
'port' becomes 'aden' (I've seen this before).
'le'+'aden'='LEADEN'
'is' acts as a link.
(Other definitions for leaden that I've seen before include "Oppressive" , "Heavy - overcast" , "Leaned over in slow, heavy way" , "Heavy - plodding" , "Slow-moving, being like heavy metal" .)