The writer is a good one, but not on metaphor (5)
I believe the answer is:
image
'metaphor' is the definition.
(thesaurus)
'the writer is a good one but not on' is the wordplay.
'the writer is' becomes 'I'm' ('I am', referring to the person who wrote the clue).
'good' becomes 'g' (abbreviation).
'but not' indicates named letters should be taken away.
'one' with 'on' taken out is 'e'.
'im'+'a'+'g'+'e'='IMAGE'
(Other definitions for image that I've seen before include "sculpture" , "Likeness; statue" , "Mental or physical representation" , "Optical counterpart in mirror" , "Mental picture; metaphor" .)