Benefit from changing ending of book (4)
I believe the answer is:
boon
'benefit' is the definition.
(I've seen this before)
'changing ending of book' is the wordplay.
I cannot really see how this works, but
'of' could be 'o' and 'o' is present in the answer.
'book' could be 'b' (abbreviation for book) and 'b' is found in the answer.
The remaining letters 'on' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'from' is the link.
(Other definitions for boon that I've seen before include "Beneficial thing" , "Something beneficial - a blessing" , "Favour, request" , "Blessing - sort of companion?" , "Gracious gift" .)