Model's terrible prose (5)
I believe the answer is:
poser
'model's' is the definition.
(synonyms)
'terrible prose' is the wordplay.
'terrible' indicates anagramming the letters.
'prose' with letters rearranged gives 'POSER'.
(Other definitions for poser that I've seen before include "Question - a sitter" , "Brain-teaser" , "Conundrum - model?" , "Puzzling question about ropes" , "Artist's model" .)