Popular poetry? The opposite (7)
I believe the answer is:
inverse
'the opposite' is the definition.
(synonyms)
'popular poetry?' is the wordplay.
'popular' becomes 'in' (something that's 'in' is fashionable).
'poetry?' becomes 'verse' (synonyms).
'in'+'verse'='INVERSE'
(Other definitions for inverse that I've seen before include "Upside down - opposite in effect" , "A direct opposite" , "Reversed in position like enviers" , "Reversed in order or relation" , "Something the exact opposite of another" .)