I'm included in an American's hostility (6)

I believe the answer is:

animus

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'hostility' is the definition.
(I know that animus is a type of hostility)

'i'm included in an american's' is the wordplay.
'included in' indicates putting letters inside.
'american's' becomes 'us' (I can't justify this - if you can you should believe this answer much more).
'an'+'us'='anus'
'im' going into 'anus' is 'ANIMUS'.

Can you help me to learn more?

(Other definitions for animus that I've seen before include "Heat" , "Deep-seated hostility or vital force" , "Ill-feeling; hatred" , "spirit of hostility" , "Motivating spirit" .)

I've seen this clue in The Globe and Mail.

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