Anyway, book is about No. 1, we read from reverse note (9)
I believe the answer is:
semibreve
'note' is the definition.
(musical note)
'anyway book is about no 1 we read from reverse' is the wordplay.
I cannot quite see how this works, but
'book' could be 'b' (abbreviation for book) and 'b' is present in the answer.
'about' could be 're' (regarding) and 're' is found in the answer.
'1' could be 'i' (Roman numeral) and 'i' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for semibreve that I've seen before include "Note equal to two minims" , "2 minims/4 crotchets" , "Note equivalent to four crotchets" .)