The French port is very heavy (6)
I believe the answer is:
Here is my best explanation:
'very heavy' is the definition.
'the french port' is the wordplay.
'the french' becomes 'le'.
'port' becomes 'aden' (I've seen this before).
'is' acts as a link.
(Other definitions for leaden that I've seen before include "Oppressive", "Heavy and slow, as if weighted down with heavy metal", "Gloomy - slow-moving", "Dull, heavy, slow", "Leaned over in slow, heavy way", "Slow like heavy metal", "Heavy - plodding", "Describes grey overcast skies", "Heavy - dull" and "Heavy - overcast".)