Not all disagreements upset Arab leader (5)
I believe the answer is:
emeer
'arab leader' is the definition.
(historical title in Islamic countries)
'not all disagreements upset' is the wordplay.
'not all' indicates a hidden word (only some of the letters).
'upset' says the letters should be written backwards.
'reeme' can be found hidden inside 'disagreements'.
'reeme' back-to-front is 'EMEER'.
(Other definitions for emeer that I've seen before include "independent ruler" , "Prince" , "Islamic title" , "Islamic governor" , "Meryl -, US actress" .)