Horse-borne Catholic major (5)
I believe the answer is:
focal
'major' is the definition.
Both the answer and definition are adjectives. Perhaps you can see a link between them that I don't see?
'horse-borne catholic' is the wordplay.
I cannot quite see how this works, but
'catholic' could be 'c' (abbreviation as in RC for Roman Catholic) and 'c' is located in the answer.
'horse' could be 'foal' (I've seen this in another clue) and 'foal' is present in the remaining letters.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for focal that I've seen before include "- length characterises a lens" , "-- Length, -- plane; -- point" , "and 21 Across: Centre of attraction" , "Relating to point of concentration" , "Relating to a central point of interest" .)