Finish with unusually sore back (7)
I believe the answer is:
endorse
'back' is the definition.
('endorse' can be a synonym of 'back')
'finish with unusually sore' is the wordplay.
'finish' becomes 'end' ('end' can be a synonym of 'finish'**).
'with' says to put letters next to each other.
'unusually' indicates an anagram.
'sore' with letters rearranged gives 'orse'.
'end'+'orse'='ENDORSE'
(Other definitions for endorse that I've seen before include "No reeds (anag)" , "Put penalty points on (a driving licence)" , "Rubber-stamp" , "Back, sanction" , "Red ones (anag.)" .)