Clue: Essential part of Trinity, one routine seen amid shimmering spires (12)
Answer: PERICHORESIS
Explanation: Definition: Essential part of Trinity
I CHORE in SPIRES* – in spectacularly unhelpful fashion, Chambers defines this with the one word "cirumincession"; in turn is defined as "the reciprocity of existence in one another of the three persons of the Trinity", which still leaves me very little the wiser. I dare say people were burned at the stake for believing or not believing in it.