Religious upset in unorthodox Celtic circle (8)
I believe the answer is:
ecliptic
'circle' is the definition.
(I've seen this before)
'religious upset in unorthodox celtic' is the wordplay.
'religious' becomes 'pi' (british abbreviation for pious).
'upset' says the letters should be written in reverse.
'in' means one lot of letters goes inside another.
'unorthodox' indicates an anagram.
'pi' reversed gives 'ip'.
'celtic' anagrammed gives 'ecltic'.
'ip' put inside 'ecltic' is 'ECLIPTIC'.
(Other definitions for ecliptic that I've seen before include "Apparent heavenly path of the sun's annual motion" , "circle" .)