Sets of vertebrae (6)
I believe the answer is:
spines
'sets of vertebrae' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for spines that I've seen before include "Thorns or backbones" , "Porcupine quills" , "Snipes at backbones" , "Quills; prickles" , "Sets of vertebrae" .)