A problem in prose composition (5)
I believe the answer is:
poser
'a problem' is the definition.
(poser is a kind of problem)
'prose composition' is the wordplay.
'composition' is an anagram indicator.
'prose' is an anagram of 'POSER'.
'in' acts as a link.
(Other definitions for poser that I've seen before include "subject for an artist?" , "Brain-teaser" , "Difficult problem" , "Someone behaving so as to be noticed" , "question master?" .)