Rivals in union: one French -- righteous, utterly -- facing Scotsman linked to church (6,8)
I believe the answer is:
Here is my best explanation:
'rivals in union' is the definition.
'one french righteous utterly facing scotsman linked to church' is the subsidiary indication.
I cannot really see how this works, but
'one' could be 'un' (I've seen this in other clues) and 'un' is present in the answer.
'righteous' could be 'holy' and 'holy' is present in the answer.
'utterly' could be 'all' and 'all' is present in the answer.
'scotsman' could be 'ian' and 'ian' is present in the answer.
'church' could be 'ce' (abbreviation for Church of England) and 'ce' is present in the answer.
No letters remain.
This explanation may well be incorrect...
(Other definitions for unholy alliance that I've seen before include "Not a church wedding", "seemingly unlikely coalition" and "Its participants don't like each other!".)