Early post I sorted consisting mainly of letters (10)

I believe the answer is:

epistolary

I'm a little stuck... Click here to teach me more about this clue!

'of letters' is the definition.
(I've seen this before)

'early post i sorted' is the wordplay.
'sorted' indicates anagramming the letters.
'early'+'post'+'i'='earlyposti'
'earlyposti' is an anagram of 'EPISTOLARY'.

'consisting mainly' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.

Can you help me to learn more?

(Other definitions for epistolary that I've seen before include "Dealing with letters" , "Church book" .)

I've seen this clue in The Independent.

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