Mostly grand manner not unknown in letter (7)
I believe the answer is:
epistle
'letter' is the definition.
(epistle is a kind of letter)
I can't explain the remainder of the clue.
(Other definitions for epistle that I've seen before include "(E.g. Paul's) letter" , "Specially long and formal letter" , "Maybe Paul's" , "A letter - as from Apostle?" , "Writing" .)