Tiny stones in backbone? (4)
I believe the answer is:
grit
This clue is a double definition.
'tiny stones' is the first definition.
'backbone?' is the second definition.
(synonyms)
'in' acts as a link.
(Other definitions for grit that I've seen before include "Determination - small stones" , "Determination - which is found on winter roads" , "Gravel or colloquial courage" , "Small hard particles - determination" , "Small loose particles of stone" .)