Purify emaciated son in church (7)
I believe the answer is:
cleanse
'purify' is the definition.
(I've seen this before)
'emaciated son in church' is the wordplay.
'emaciated' becomes 'lean' (similar in meaning).
'son' becomes 's' (genealogical abbreviation for son).
'in' means one lot of letters goes inside another.
'church' becomes 'CE' (abbreviation for Church of England).
'lean'+'s'='leans'
'leans' put inside 'ce' is 'CLEANSE'.
(Other definitions for cleanse that I've seen before include "give absolution" , "Free of sin" , "Purge" , "Thoroughly remove dirt from" , "Make pure" .)