Fatuous writer makes a comeback in it (5)
I believe the answer is:
inept
'fatuous' is the definition.
(thesaurus)
'writer makes a comeback in it' is the wordplay.
'writer' becomes 'pen' (something which writes).
'makes a comeback' says the letters should be written backwards (I've seen 'making a comeback' mean this).
'in' indicates putting letters inside.
'pen' backwards is 'nep'.
'nep' placed within 'it' is 'INEPT'.
(Other definitions for inept that I've seen before include "Unskilful or unsuitable" , "Wanting" , "Incompetent or awkward" , "Maladroit" , "Hopeless; bungling" .)