A poem set around non-fashion (6)
I believe the answer is:
sonnet
'a poem' is the definition.
(type of poem)
'set around non-fashion' is the wordplay.
'around' means one lot of letters goes inside another.
'fashion' indicates anagramming the letters.
'non' anagrammed gives 'onn'.
'set' placed around 'onn' is 'SONNET'.
(Other definitions for sonnet that I've seen before include "Fourteen line poem" , "Form of poem" , "A form of poetry" , "Poem of 14 lines - Shakespeare wrote lots" , "14-line verse" .)