What reader did at the end possibly is a mystery (6,4)
I believe the answer is:
closed book
'mystery' is the definition.
(I've seen this before)
'what reader did at the end possibly' is the wordplay.
I cannot really see how this works, but
'end' could be 'close' (close can mean the closing part of something) and 'close' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
'is a' is the link.
Can you help me to learn more?
(Other definitions for closed book that I've seen before include "Subject about which one knows nothing" , "Mystery person" , "Enigma" , "Took no more bets" , "What loss-making turf accountant did" .)