D’Artagnan, for instance, could talk a lot and fool people (6)
I believe the answer is:
gascon
'd'artagnan for instance' is the definition.
I can't judge whether this definition defines the answer.
'talk a lot and fool people' is the wordplay.
'talk' becomes 'gas' (I've seen this before).
'a lot' becomes 'c'.
'and' is a charade indicator (letters next to each other).
'fool people' becomes 'on' (I can't justify this - if you can you should give a lot more credence to this answer).
'gas'+'c'+'on'='GASCON'
'could' is the link.
Can you help me to learn more?
(Another definition for gascon that I've seen is " French dialect".)