Operating on a scar reveals bones (5)
I believe the answer is:
sacra
'bones' is the definition.
(sacrum is a kind of bone)
'operating on a scar' is the wordplay.
'operating' is an anagram indicator.
'on' means one lot of letters go next to another.
'a' after 'scar' is 'scara'.
'scara' is an anagram of 'SACRA'.
'reveals' is the link.
(Other definitions for sacra that I've seen before include "Triangular bones" , "Bones that form the human pelvic arch" .)