Finish with terribly sore back (7)
I believe the answer is:
endorse
'back' is the definition.
('endorse' can be a synonym of 'back')
'finish with terribly sore' is the wordplay.
'finish' becomes 'end' ('end' can be a synonym of 'finish'**).
'with' is a charade indicator (letters next to each other).
'terribly' is an anagram indicator.
'sore' anagrammed gives 'orse'.
'end'+'orse'='ENDORSE'
(Other definitions for endorse that I've seen before include "Give public approval of (something)" , "Give support or approval to" , "Support (with signature)" , "Approve officially" , "Approve of, sign on back as of cheque" .)