Sound level for book (6)
I believe the answer is:
volume
This clue could be a double definition.
'sound level' is the first definition.
'volume' can be an answer for 'sound' (I've seen this before). I'm unsure of the 'level' bit.
'for book' is the second definition.
(synonyms)
Can you help me to learn more?
(Other definitions for volume that I've seen before include "reading matter?" , "Loud sound or big book" , "Control of radio" , "Book - sound level" , "that listeners may turn down" .)