Bless ancestor's reformation in church (10)
I believe the answer is:
consecrate
'bless' is the definition.
('consecrate' can be a synonym of 'bless')
'ancestor's reformation in church' is the wordplay.
'reformation' indicates an anagram.
'in' is an insertion indicator.
'church' becomes 'ce' (abbreviation for Church of England).
'ancestor' is an anagram of 'onsecrat'.
'onsecrat' placed into 'ce' is 'CONSECRATE'.
(Other definitions for consecrate that I've seen before include "Bless" , "Make 18 [SACRED]" , "giving cause to be respected" , "Sanctify" , "Make holy" .)