Book of French in the back end (6)
I believe the answer is:
reader
'book' is the definition.
(reader is a kind of book)
'of french in the back end' is the wordplay.
'of french' becomes 'de' ('of' in French).
'in the' indicates putting letters inside.
'back end' becomes 'rear' (synonyms).
'de' inserted within 'rear' is 'READER'.
(Other definitions for reader that I've seen before include "University lecturer - text book" , "One perusing book" , "Senior lecturer" , "Person perusing a book or newspaper" , "Kind of textbook - or its peruser" .)