Setbacks concerning poetic endeavours (8)
I believe the answer is:
reverses
'setbacks' is the definition.
(I know this)
'concerning poetic endeavours' is the wordplay.
'concerning' becomes 're' (both can mean on a particular topic).
'poetic endeavours' becomes 'verses' (lines of poetry).
're'+'verses'='REVERSES'
(Other definitions for reverses that I've seen before include "Backs up" , "Defeats" , "Setbacks" , "Moves backwards" , "Goes backwards" .)