Endlessly evil book returned as contentious (7)
I believe the answer is:
emotive
'contentious' is the definition.
(I've seen this before)
'endlessly evil book returned' is the wordplay.
'endlessly' means to remove the last letter.
'book' becomes 'tome' (a tome is a book).
'returned' says the letters should be written backwards.
'evil' with its final letter taken away is 'evi'.
'evi'+'tome'='evitome'
'evitome' reversed gives 'EMOTIVE'.
'as' acts as a link.
(Other definitions for emotive that I've seen before include "Arousing deep feeling" , "Highly-charged" , "Stirring feelings" , "Arousing intense feelings" , "Showing feelings" .)