Argentine fossil vertebrae exhibits (6)
I believe the answer is:
silver
'argentine' is the definition.
(thesaurus)
'fossil vertebrae exhibits' is the wordplay.
'exhibits' says the answer is hidden in the clue.
'SILVER' is hidden within 'fossil vertebrae'.
(Other definitions for silver that I've seen before include "See 20" , "symbolically a twenty-fifth" , "Pirate" , "Precious stuff" , "Number forty-seven" .)