Two churches seizing an opportunity (6)
I believe the answer is:
chance
'opportunity' is the definition.
(I know that opportunity can be written as chance)
'two churches seizing an' is the wordplay.
'two churches' means two replacements for 'church'.
'church' becomes 'ch' (common abbreviation).
'church' becomes 'CE' (abbreviation for Church of England).
'seizing' means one lot of letters goes inside another (inserted letters are seized).
'ch'+'ce'='chce'
'chce' going around 'an' is 'CHANCE'.
(Other definitions for chance that I've seen before include "Odds" , "Liability" , "Accidental; opportunity" , "Opportunity; random effect" , "Possibility of something happening" .)