Purge cant spoken principally in church (7)
I believe the answer is:
cleanse
'purge' is the definition.
(I've seen this before)
'cant spoken principally in church' is the wordplay.
'cant' becomes 'lean' (I've seen this in another clue).
'principally' says to take the initial letters.
'in' indicates putting letters inside.
'church' becomes 'CE' (abbreviation for Church of England).
The first letter of 'spoken' is 's'.
'lean'+'s'='leans'
'leans' inserted into 'ce' is 'CLEANSE'.
(Other definitions for cleanse that I've seen before include "give absolution" , "Make thoroughly clean" , "Thoroughly remove dirt from" , "Make pure" , "Make spotless" .)