Star, a Catholic principally tracking aurochs (8)
I believe the answer is:
arcturus
'star' is the definition.
(I know that Arcturus is or was a giant)
'a catholic principally tracking aurochs' is the wordplay.
'catholic' becomes 'RC' (abbreviation for Roman Catholic).
'principally' indicates taking the first letters.
'aurochs' becomes 'urus' (synonyms).
The first letter of 'tracking' is 't'.
'a'+'rc'+'t'+'urus'='ARCTURUS'
(Other definitions for arcturus that I've seen before include "Very bright star" , "first-magnitude star" , "One of the brightest stars" , "Bright red giant" , "Star (the Bearward)" .)