Main US source of hostility (6)
I believe the answer is:
animus
'hostility' is the definition.
(I know that animus is a type of hostility)
'main us source of' is the wordplay.
'source of' indicates anagramming the letters.
'main'+'us'='mainus'
'mainus' is an anagram of 'ANIMUS'.
(Other definitions for animus that I've seen before include "US man I turn to with hostility" , "Ill-feeling; hatred" , "A feeling of ill will or hostility" , "Hostility, bad feeling toward" , "Hostile spirit in speech or action" .)