Animus (7)
'animus' is the definition.
(thesaurus)
This is all the clue.
(Other definitions for ill will that I've seen before include "causing resentment" , "Hostility, malevolence" , "Bad feeling towards, hostility" , "Rancour" , "Animosity, hostile feeling" .)
(Other definitions for illwill that I've seen before include "Hatred" , "bad blood" .)