Piece written in English — a line of letters (10)
I believe the answer is:
epistolary
'of letters' is the definition.
(I've seen this before)
'piece written in english a line' is the wordplay.
'piece' becomes 'pistol' (pistol is a kind of piece).
'written in' is an insertion indicator.
'english' becomes 'e' (abbreviation).
'line' becomes 'ry' (abbreviation for railway).
'e'+'a'='ea'
'pistol' placed inside 'ea' is 'epistola'.
'epistola'+'ry'='EPISTOLARY'
(Other definitions for epistolary that I've seen before include "Church book" , "Dealing with letters" .)