Key associate of Falstaff in charge of letter-writing (9)

I believe the answer is:

epistolic

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'letter-writing' is the definition.
Both the definition and answer are adjectives.
Perhaps they are linked in a way I don't understand?

'key associate of falstaff in charge' is the wordplay.
'key' becomes 'e' (a musical key).
'associate of falstaff' becomes 'pistol' (I can't justify this - if you can you should give a lot more credence to this answer).
'in charge' becomes 'ic' (abbreviation for 'in charge').
'e'+'pistol'+'ic'='EPISTOLIC'

'of' acts as a link.

Can you help me to learn more?

I've seen this clue in The Times.

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