High priest's power over quarrel (7)
I believe the answer is:
pontiff
'high priest's' is the definition.
(thesaurus)
'power over quarrel' is the wordplay.
'power' becomes 'p'.
'over' becomes 'on' (I've seen this in other clues**).
'quarrel' becomes 'tiff' (tiff is a kind of quarrel).
'p'+'on'+'tiff'='PONTIFF'
(Other definitions for pontiff that I've seen before include "High-priest" , "Head of the Roman Catholic church" , "Religious leader" , "Title for a pope" , "Bishop" .)