Most fit a book in case (6)
I believe the answer is:
ablest
'most fit' is the definition.
('able' can be similar in meaning to 'fit')
'a book in case' is the wordplay.
'book' becomes 'b' (abbreviation for book).
'in case' becomes 'lest' (eg 'lest we forget').
'a'+'b'+'lest'='ABLEST'
(Other definitions for ablest that I've seen before include "Most suitable" , "Like pupils in top stream" , "Are highly favoured like" , "Most capable" , "Most proficient" .)