Bones used in holy rituals, mostly (5)
I believe the answer is:
sacra
'bones' is the definition.
(sacrum is a kind of bone)
'used in holy rituals mostly' is the wordplay.
'used in holy rituals' becomes 'sacral' (relating to sacred rituals).
'mostly' means to remove the last letter.
'sacral' with its last letter removed is 'SACRA'.
(Other definitions for sacra that I've seen before include "Triangular bones" , "Bones that form the human pelvic arch" .)