Support back, terribly sore (7)
I believe the answer is:
endorse
'support' is the definition.
(synonyms)
'back terribly sore' is the wordplay.
'back' becomes 'end' (I've seen this in other clues).
'terribly' indicates anagramming the letters.
'sore' anagrammed gives 'orse'.
'end'+'orse'='ENDORSE'
(Other definitions for endorse that I've seen before include "Champion" , "Back, sanction" , "Say you approve" , "Favour" , "Sore end (anag.)" .)