Engineers in, supporting church buttress (9)
I believe the answer is:
reinforce
'buttress' is the definition.
(thesaurus)
'engineers in supporting church' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'supporting' becomes 'for'.
'church' becomes 'ce' (abbreviation for Church of England).
're'+'in'+'for'+'ce'='REINFORCE'
(Other definitions for reinforce that I've seen before include "Give added strength to" , "increase support" , "give added support" , "provide support" , "Strengthen or support with confrerie" .)