Subject not understood in finished novel? (6,4)
I believe the answer is:
closed book
'subject not understood' is the definition.
(a closed book is something not understood)
'finished novel?' is the wordplay.
'finished' becomes 'closed' (closing is a kind of finishing).
'novel?' becomes 'book' (novel is a kind of book).
'closed'+'book'='CLOSED BOOK'
'in' acts as a link.
(Other definitions for closed book that I've seen before include "Enigma" , "What loss-making turf accountant did" , "Took no more bets" , "Mystery person" , "Subject about which one knows nothing" .)