Sweetener upset team following a dispute (9)
I believe the answer is:
aspartame
'sweetener' is the definition.
(aspartame is a kind of sweetener)
'upset team following a dispute' is the wordplay.
'upset' indicates anagramming the letters.
'following' means one lot of letters go next to another.
'dispute' becomes 'spar' (I've seen this before).
'team' with letters rearranged gives 'tame'.
'a'+'spar'='aspar'
'tame' put after 'aspar' is 'ASPARTAME'.
(Other definitions for aspartame that I've seen before include "What sweetens" , "Sugar substitute" , "Strong artificial sweetener -- same apart (anag)" .)