Writer’s function and aim (9)
I believe the answer is:
ballpoint
'writer's' is the definition.
(writing implement)
'function and aim' is the wordplay.
'function' becomes 'ball' (ball can mean a party or function).
'and' says to put letters next to each other.
'aim' becomes 'point' (pointing is a kind of aiming).
'ball'+'point'='BALLPOINT'
(Other definitions for ballpoint that I've seen before include "Writer" , "Biro" , "Type of pen" .)