Religious leader is quiet about a disagreement (7)
I believe the answer is:
pontiff
'religious leader' is the definition.
(I've seen this before)
'quiet about a disagreement' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'about' becomes 'on' (as in a book 'on' a certain topic).
'a disagreement' becomes 'tiff' (tiff is a kind of disagreement).
'p'+'on'+'tiff'='PONTIFF'
'is' acts as a link.
(Other definitions for pontiff that I've seen before include "Religious leader" , "Head of the Roman Catholic church" , "Title for a pope" , "High-priest" , "Bishop" .)