Friend had upset spiritual leader (6)
I believe the answer is:
buddha
'spiritual leader' is the definition.
'buddha' can be an answer for 'leader' (I've seen this before). I am unsure of the 'spiritual' bit.
'friend had upset' is the wordplay.
'friend' becomes 'bud' (shortly for buddy).
'upset' indicates anagramming the letters.
'had' is an anagram of 'dha'.
'bud'+'dha'='BUDDHA'
Can you help me to learn more?
(Other definitions for buddha that I've seen before include "religious picture" , "Aka Siddhartha Gautama" , "Siddhartha Gautama (c 563-483 BC)" , "The philosopher Gautama who founded a great eastern religion" , "Gautama . . . . . . 5th/6th century Asian philosopher on whom a religion is based" .)