Book, as the case may be, fine (6,4)
I believe the answer is:
vanity fair
'book' is the definition.
(I've seen this before)
'the case may be fine' is the wordplay.
'the case may' becomes 'vanity' (I can't explain this - if you can you should believe this answer much more).
'be fine' becomes 'fair' (I've seen this before).
'vanity'+'fair'='VANITY FAIR'
'as' is the link.
Can you help me to learn more?
(Other definitions for vanity fair that I've seen before include "Book by Thackeray" , "Sharp tale" , "Novel by Thackeray" , "place visited by Christian" , "market pilgrim progressed through" .)