In favour of state retaining one constant feature of church architecture? (11)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

FOR (in favour of) + NATION (state), containing I (one in Roman numerals) + C (abbreviation for a mathematical constant). It means arched structures, such as those holding up the roof of a traditional church building. Any other interpretation is your own business.  (But the derivation seems to be from the Latin fornix for a vault with an arched roof, which then suggests an underground hideaway where you can meet someone in secret.)