Writer about to back revolutionary book's sentiment (7)
I believe the answer is:
Here is my best explanation:
'sentiment' is the definition.
'writer about to back revolutionary book's' is the subsidiary indication.
'writer' becomes 'me' (the person who is writing these clues).
'about' shows that the letters should be reversed in order.
'back' says the letters should be written in reverse.
'revolutionary book' becomes 'ion' (I can't justify this - if you can you should give a lot more credence to this answer).
'me' back-to-front is 'em'.
'to' in reverse letter order is 'ot'.
(Other definitions for emotion that I've seen before include "A feeling - love, hate etc", "Feeling - sentiment", "A strong instinctive feeling", "Hatred or love, say", "Feeling like love or fear", "Feeling such as joy, sorrow and fear", "Any strong feeling", "reaction", "Intuitive feeling" and "Strong feeling of joy, fear etc".)