Mystery of blocked reserve (6,4)
I believe the answer is:
closed book
'mystery' is the definition.
(I've seen this before)
'blocked reserve' is the wordplay.
'blocked' becomes 'closed' (similar in meaning).
'reserve' becomes 'book' (to book something is to reserve it).
'closed'+'book'='CLOSED BOOK'
'of' is the link.
(Other definitions for closed book that I've seen before include "Took no more bets" , "Enigma" , "Subject about which one knows nothing" , "What loss-making turf accountant did" , "Mystery person" .)