Poser set in unpoetical language (5)
I believe the answer is:
prose
'unpoetical language' is the definition.
'prose' can be an answer for 'language' (I have seen 'Ordinary language ' mean 'prose' so perhaps 'language' could also mean 'prose'). I am not certain of the 'unpoetical' bit.
'poser set' is the wordplay.
'set' is an anagram indicator (instruction to set the letters).
'poser' with letters rearranged gives 'PROSE'.
'in' is the link.
Can you help me to learn more?
(Other definitions for prose that I've seen before include "Gossipy talker" , "Ordinary spoken or written language" , "Non-verse" , "Normal written language" , "Writings not in verse" .)