Attacker of Alsatians beaten up (9)
I believe the answer is:
assailant
'attacker' is the definition.
('assailant' can be a synonym of 'attacker')
'alsatians beaten up' is the wordplay.
'beaten up' indicates anagramming the letters.
'alsatians' with letters rearranged gives 'ASSAILANT'.
'of' is the link.
(Other definitions for assailant that I've seen before include "Aggressive sort" , "Aggressor" , "Attacking person" , "who is on the offensive?" , "One attacking another" .)