Alsatians let loose to find aggressor (9)
I believe the answer is:
assailant
'aggressor' is the definition.
(I've seen this before)
'alsatians let loose' is the wordplay.
'let loose' indicates an anagram.
'alsatians' anagrammed gives 'ASSAILANT'.
'to find' is the link.
(Other definitions for assailant that I've seen before include "Aggressor" , "An attacker like mugger" , "One attacking another" , "Aggressive sort" , "One attacks" .)