Problem in composing prose (5)
I believe the answer is:
poser
'problem' is the definition.
(I know that poser is a type of problem)
'composing prose' is the wordplay.
'composing' indicates an anagram.
'prose' with letters rearranged gives 'POSER'.
'in' acts as a link.
(Other definitions for poser that I've seen before include "Puzzling question about ropes" , "Artist's model" , "Ropes (anag.)" , "Toughie" , "Tricky clue" .)