Puritan Mass said first (8)
I believe the answer is:
moralist
'puritan' is the definition.
(both can mean a sanctimonious person)
'mass said first' is the wordplay.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'said' becomes 'oral' (I've seen this before).
'first' becomes 'ist' (ist resembles 1st).
'm'+'oral'+'ist'='MORALIST'
(Other definitions for moralist that I've seen before include "Principled type" , "puritan" , "someone drawing lessons" , "Ail storm (anag)" , "Disciplinarian" .)