Bones used in holy rituals, mostly (5)

I haven't cracked this one yet — but the brilliant team at Times for the Times have:

SACRA {ment} (holy rituals) [mostly]. I was puzzled by the parsing here because if you equate plural with plural (‘rituals’ with ‘sacraments’) the deletion indicator ‘mostly’ wouldn’t be appropriate. However if you take it that a single sacrament consists of a number of rituals then the deletion works 5 to 4. Edit: I’m not going to delete my version because it works (sort of) but thanks to melrosemike for pointing out that the more likely parsing is: SACRA {l} (used in holy rituals) [mostly].