Terrible prose by impostor (5)
I believe the answer is:
poser
'impostor' is the definition.
(thesaurus)
'terrible prose' is the wordplay.
'terrible' indicates an anagram.
'prose' anagrammed gives 'POSER'.
'by' acts as a link.
(Other definitions for poser that I've seen before include "Tricky clue" , "Show-off" , "difficult puzzle" , "Particularly baffling problem" , "Problem to be solved" .)