Reverence is adopted by head in old church (9)
I believe the answer is:
obeisance
'reverence' is the definition.
(thesaurus)
'is adopted by head in old church' is the wordplay.
'adopted by' indicates putting letters inside (I've seen this in other clues).
'head' becomes 'bean' (bean is a kind of head**).
'in' is an insertion indicator.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'church' becomes 'ce' (abbreviation for Church of England).
'is' placed inside 'bean' is 'beisan'.
'o'+'ce'='oce'
'beisan' going inside 'oce' is 'OBEISANCE'.
(Other definitions for obeisance that I've seen before include "Act of submission" , "Bowing" , "curtsy" , "Respectful gesture" , "Homage or deference" .)