Alsatians upset the attacker (9)
I believe the answer is:
assailant
'the attacker' is the definition.
('assailant' can be a synonym of 'attacker')
'alsatians upset' is the wordplay.
'upset' indicates anagramming the letters.
'alsatians' anagrammed gives 'ASSAILANT'.
(Other definitions for assailant that I've seen before include "Aggressive sort" , "Who would tear into" , "An attacker like mugger" , "One attacks" , "Aggressor" .)