Book shelf's right underneath (6)
I believe the answer is:
ledger
'book' is the definition.
(I know that book can be written as ledger)
'shelf's right underneath' is the wordplay.
'shelf' becomes 'ledge' (synonyms).
'right' becomes 'r' (common abbreviation).
'underneath' is a charade indicator (letters next to each other) (in a down clue, some letters go under others).
'ledge'+'r'='LEDGER'
(Other definitions for ledger that I've seen before include "Elderly resident" , "In which accounts are entered" , "Heath -, Australian actor" , "Book for keeping financial accounts" , "Accounting journal" .)