Book right behind sill (6)
I believe the answer is:
ledger
'book' is the definition.
(ledger is a kind of book)
'right behind sill' is the wordplay.
'right' becomes 'r' (common abbreviation).
'behind' is a charade indicator (letters next to each other).
'sill' becomes 'ledge' (I've seen this before).
'r' after 'ledge' is 'LEDGER'.
(Other definitions for ledger that I've seen before include "Elderly resident" , "In which accounts are entered" , "Book-keeper's book" , "Heath -, Australian actor" , "Accounting journal" .)