Celtic pair regularly playing in circle (8)
I believe the answer is:
ecliptic
'circle' is the definition.
(thesaurus)
'celtic pair regularly playing' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'playing' indicates an anagram.
The alternate letters of 'pair' are 'pi'.
'celtic'+'pi'='celticpi'
'celticpi' with letters rearranged gives 'ECLIPTIC'.
'in' acts as a link.
(Other definitions for ecliptic that I've seen before include "Apparent heavenly path of the sun's annual motion" , "circle" .)