Model’s terrible prose (5)
I believe the answer is:
'model's' is the definition.
'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" .)