Factor is contributing to separation abandoned by church (7)
I believe the answer is:
divisor
'factor' is the definition.
(I know that factor can be written as divisor)
'is contributing to separation abandoned by church' is the wordplay.
I cannot quite see how this works, but
'is' is within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for divisor that I've seen before include "Sort of number" , "factor" , "B in A/B" , "Number under line" , "It will go in exactly" .)