Writing out the final chapter, makes improvements (5)
I believe the answer is:
mends
'improvements' is the definition.
Although both the answer and definition are plural nouns, I don't see how they can define each other.
'writing out the final chapter' is the wordplay.
'writing' becomes 'ms' (abbreviation for manuscript).
'out' is an insertion indicator.
'the final chapter' becomes 'end' (I've seen this in another clue).
'ms' going around 'end' is 'MENDS'.
'makes' acts as a link.
Can you help me to learn more?
(Other definitions for mends that I've seen before include "Repairs, darns" , "Fixes" .)