Fashionable poetry is upside down (7)
I believe the answer is:
inverse
'is upside down' is the definition.
(I've seen this before)
'fashionable poetry' is the wordplay.
'fashionable' becomes 'in' (in can mean fashionable or trendy).
'poetry' becomes 'verse' (lines written in verse).
'in'+'verse'='INVERSE'
(Other definitions for inverse that I've seen before include "A direct opposite" , "Antonym" , "Reversed in order or relation" , "Opposite (like poetry?)" , "Reversed in position like enviers" .)