Backbone of copper or tin, say (6)
I believe the answer is:
mettle
'backbone' is the definition.
(thesaurus)
'copper or tin say' is the wordplay.
'copper or tin' becomes 'metal' (I've seen this in another clue).
'say' shows a homophone (sound like).
'metal' is a homophone of 'METTLE'.
'of' acts as a link.
(Other definitions for mettle that I've seen before include "The courage to carry on" , "Quality" , "boldness" , "Fortitude" , "Strength of character, courage" .)