Done reading mystery (6,4)
I believe the answer is:
closed book
'mystery' is the definition.
(I've seen this before)
'done reading' is the wordplay.
'done' becomes 'closed' (I can't explain this - if you can you should give a lot more credence to this answer).
'reading' becomes 'book'.
'closed'+'book'='CLOSED BOOK'
Can you help me to learn more?
(Other definitions for closed book that I've seen before include "Mystery person" , "What loss-making turf accountant did" , "Subject about which one knows nothing" , "Enigma" , "Took no more bets" .)