Buttress supporting church -- it's under control (9)
I believe the answer is:
reinforce
'buttress' is the definition.
(thesaurus)
'supporting church it's under control' is the wordplay.
'supporting' becomes 'for'.
'church' becomes 'ce' (abbreviation for Church of England).
'it's under' means one lot of letters go next to another.
'control' becomes 'rein' (rein is a kind of control).
'for'+'ce'='force'
'force' put after 'rein' is 'REINFORCE'.
(Other definitions for reinforce that I've seen before include "provide support" , "Buttress" , "give added support" , "Give added strength to" , "Strengthen or support with confrerie" .)