Back possibly sore after final (7)
I believe the answer is:
endorse
'back' is the definition.
('endorse' can be a synonym of 'back')
'possibly sore after final' is the wordplay.
'possibly' indicates an anagram.
'after' is a charade indicator (letters next to each other).
'final' becomes 'end' ('end' can be a synonym of 'final'**).
'sore' with letters rearranged gives 'orse'.
'orse' after 'end' is 'ENDORSE'.
(Other definitions for endorse that I've seen before include "Docket" , "Sanction -- red nose (anag)" , "Favour" , "Support; sanction" , "Give one's sanction to" .)