Rivals in union: one French -- righteous, utterly -- facing Scotsman linked to church (6,8)
I believe the answer is:
unholy alliance
'rivals in union' is the definition.
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for unholy alliance that I've seen before include "seemingly unlikely coalition" , "Not a church wedding" , "Its participants don't like each other!" .)