Model’s terrible prose (5)
I believe the answer is:
poser
'model's' is the definition.
(synonyms)
'terrible prose' is the wordplay.
'terrible' indicates an anagram.
'prose' is an anagram of '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" .)