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