Book right at the back of the shelf (6)
I believe the answer is:
ledger
'book' is the definition.
(I know that book can be written as ledger)
'right at the back of the shelf' is the wordplay.
'right' becomes 'r' (common abbreviation).
'at the back of' is a charade indicator (letters next to each other).
'the shelf' becomes 'ledge' (synonyms).
'r' put after 'ledge' is 'LEDGER'.
(Other definitions for ledger that I've seen before include "In which accounts are entered" , "Business record book" , "Heath -, Australian actor" , "Book for keeping financial accounts" , "Elderly resident" .)