A problem in prose, possibly (5)
I believe the answer is:
poser
'a problem' is the definition.
(I know that poser is a type of problem)
'prose possibly' is the wordplay.
'possibly' is an anagram indicator.
'prose' anagrammed gives 'POSER'.
'in' acts as a link.
(Other definitions for poser that I've seen before include "Toughie" , "One pretends to be something they are not" , "Question - a sitter" , "insincere?" , "question master?" .)