Problem expressed in prose, perhaps (5)
I believe the answer is:
poser
'problem' is the definition.
(I know that poser is a type of problem)
'prose perhaps' is the wordplay.
'perhaps' indicates anagramming the letters.
'prose' with letters rearranged gives 'POSER'.
'expressed in' is the link.
(Other definitions for poser that I've seen before include "Show-off" , "Puzzling question for the model" , "Exhibitionist" , "Conundrum - model?" , "one who has airs and graces" .)