Church diocese backing ecumenicalism from the start in Gloucester (6)
I believe the answer is:
cheese
'gloucester' is the definition.
(thesaurus)
'church diocese backing ecumenicalism from the start' is the wordplay.
'church' becomes 'ch' (common abbreviation).
'diocese' becomes 'see' (I've seen this before).
'backing' shows that the letters should be reversed in order.
'from the start' suggests taking the first letters.
The initial letter of 'ecumenicalism' is 'e'.
'see' in reverse letter order is 'ees'.
'ch'+'ees'+'e'='CHEESE'
'in' acts as a link.
(Other definitions for cheese that I've seen before include "Edam or cheddar, say" , "Big one's important" , "Lancashire, perhaps" , "Brie or Emental, say" , "Gloucester or Edam, say" .)