Useless writer is shown up in it (5)
I believe the answer is:
inept
'useless' is the definition.
(thesaurus)
'writer is shown up in it' is the wordplay.
'writer' becomes 'pen' (I've seen this before).
'is shown up' says the letters should be written in reverse.
'in' indicates putting letters inside.
'pen' reversed gives 'nep'.
'nep' inserted into 'it' is 'INEPT'.
(Other definitions for inept that I've seen before include "Foolish" , "Futile" , "Awkward, clumsy" , "Lousy" , "Wanting" .)