Spite of archdeacon initially over Mass (5)
I believe the answer is:
venom
'spite' is the definition.
(I know that spite can be written as venom)
'archdeacon initially over mass' is the wordplay.
'archdeacon' becomes 'ven' (abbreviation for The Venerable).
'initially' indicates taking the first letters.
The first letters of 'over mass' is 'om'.
'ven'+'om'='VENOM'
'of' acts as a link.
(Other definitions for venom that I've seen before include "Toxic substance" , "Snake poison" , "acrimony" , "Spite: toxin" , "ill will" .)