Very formal leaders of priesthood rotated in mass (4)
I believe the answer is:
prim
'very formal' is the definition.
(I've seen this before)
'leaders of priesthood rotated in mass' is the wordplay.
'leaders of' suggests taking the first letters.
The first letters of 'priesthood rotated in mass' is 'PRIM'.
(Other definitions for prim that I've seen before include "Straight-laced" , "Demure" , "Proper or prissy" , "Stiffly formal and proper" , "Strait-laced - prudish" .)