Lose CD cover and volume will become an unknowable thing (6,4)

I believe the answer is:

closed book

I'm a little stuck... Click here to teach me more about this clue!

'an unknowable thing' is the definition.
The definition suggests a singular noun which matches the answer.

'lose cd cover and volume' is the wordplay.
'cover' means one lot of letters goes inside another.
'and' means one lot of letters go next to another.
'volume' becomes 'book' (synonyms).
'lose' going into 'cd' is 'closed'.
'closed'+'book'='CLOSED BOOK'

'will become' is the link.

Can you help me to learn more?

(Other definitions for closed book that I've seen before include "Took no more bets" , "What loss-making turf accountant did" , "Subject about which one knows nothing" , "Mystery person" , "Enigma" .)

I've seen this clue in the Sydney Morning Herald.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.