Brave tree doctor shows some backbone (9)
I believe the answer is:
vertebrae
'some backbone' is the definition.
(thesaurus)
'brave tree doctor' is the wordplay.
'doctor' indicates anagramming the letters.
'brave'+'tree'='bravetree'
'bravetree' anagrammed gives 'VERTEBRAE'.
'shows' acts as a link.
(Other definitions for vertebrae that I've seen before include "Backbone segments" , "Bony segments of the spinal column" , "Bones of back" , "Bits of skeleton" , "Bones of the spinal column" .)