Improved writer divides group (9)
I believe the answer is:
sharpened
'improved' is the definition.
Both the answer and definition are adjectives. Perhaps you can see an association between them that I can't see?
'writer divides group' is the wordplay.
'writer' becomes 'pen' (something which writes).
'divides' means one lot of letters goes inside another (inserted letters divide the word).
'group' becomes 'shared' (I can't explain this - if you can you should believe this answer much more).
'pen' inserted within 'shared' is 'SHARPENED'.
Can you help me to learn more?
(Another definition for sharpened that I've seen is " with a point".)