Pattern attending in church (8)
I believe the answer is:
template
'pattern' is the definition.
(I know that pattern can be written as template)
'attending in church' is the wordplay.
'attending' becomes 'at' (e.g. at school means attending school).
'in' indicates putting letters inside.
'church' becomes 'temple' (I've seen this before).
'at' inserted within 'temple' is 'TEMPLATE'.
(Other definitions for template that I've seen before include "Model or standard for making comparisons" , "original design" , "Model, example to follow" , "Design model" , "Shape serving as model" .)