Come first in French, which is hard to pick up (6)
I believe the answer is:
'is hard to pick up' is the definition.
(I've seen this in another clue)
'come first in french' is the wordplay.
I cannot really understand how this works, but
'first' could be 'lead' (lead can mean leading or primary) and 'lead' is found in the answer.
'in french' could be 'en' ('in' in French) and 'en' is located in the answer.
No letters remain.
This explanation may well be incorrect...
'which' is the link.
Can you help me to learn more?
(Other definitions for leaden that I've seen before include "Heavy or slow, like dark-gray metal", "Describes grey overcast skies", "Heavy - overcast", "Heavy - plodding", "Dull, heavy, slow".)