Back on form in poetry (7)
I believe the answer is:
reverse
'back' is the definition.
(back can mean to reverse)
'on form in poetry' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'form in poetry' becomes 'verse' (verse form).
're'+'verse'='REVERSE'
(Other definitions for reverse that I've seen before include "Tails (of a coin)" , "Invert" , "Severer (anag.)" , "come back" , "Upset" .)