He is hostile letting out Alsatians (9)
I believe the answer is:
assailant
'he is hostile letting' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'out alsatians' is the wordplay.
'out' indicates an anagram.
'alsatians' anagrammed gives 'ASSAILANT'.
Can you help me to learn more?
(Other definitions for assailant that I've seen before include "One attacks" , "Attacking person" , "Aggressor" , "One attacking another" , "Who would tear into" .)