One church head needed for White Mass? (6)
I believe the answer is:
icecap
'white mass?' is the definition.
(mass of ice and snow)
'one church head needed' is the wordplay.
'one' becomes 'i' (Roman numeral).
'church' becomes 'CE' (abbreviation for Church of England).
'head' becomes 'cap' (both can mean the top of something).
'needed' means one lot of letters go next to another.
'i'+'ce'+'cap'='ICECAP'
'for' acts as a link.
(Other definitions for icecap that I've seen before include "Frozen polar area" , "Feature of polar regions" , "Permanent frozen polar cover" , "Permanent covering of frozen water" , "what's on mountain peak?" .)