Holy mountain saint favoured: excellent (5)
I believe the answer is:
sinai
'holy mountain' is the definition.
'sinai' can be an answer for 'mountain' (I've seen this before). I'm unsure of the 'holy' bit.
'saint favoured excellent' is the wordplay.
'saint' becomes 's'.
'favoured' becomes 'in' (I've seen this in other clues).
'excellent' becomes 'ai' (resembles 'A1', old-fashioned term for excellent).
's'+'in'+'ai'='SINAI'
Can you help me to learn more?
(Other definitions for sinai that I've seen before include "Peninsula, mostly desert, at north end of Red Sea" , "Mt. where Moses received 10 Commandments" , "11 Across peninsula" , "Mountainous Egyptian peninsula" , "Part of 10 [EGYPT]" .)