On reflection, reject writer (3)
I believe the answer is:
nib
'writer' is the definition.
(something which writes)
'on reflection reject' is the wordplay.
'on reflection' says the letters should be written backwards.
'reject' becomes 'bin' (I've seen this in other clues).
'bin' in reverse letter order is 'NIB'.
(Other definitions for nib that I've seen before include "Pen-point" , "Pen end" , "Writing tip of old pen" , "writer" , "Writing point of old pen" .)