Doreen's bad back (7)
I believe the answer is:
endorse
'back' is the definition.
(back can mean to support or endorse)
'doreen's bad' is the wordplay.
'bad' indicates anagramming the letters.
'doreens' is an anagram of 'ENDORSE'.
(Other definitions for endorse that I've seen before include "Champion" , "No reeds (anag)" , "Say you approve" , "Corroborate" , "Give support or approval to" .)