Sadly so ill in US, having hallucinations (9)

I believe the answer is:

illusions

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'hallucinations' is the definition.
The answer and definition can be both related to thinking as well as being plural nouns.
Maybe there's a link between them I don't understand?

'sadly so ill in us' is the wordplay.
'sadly' indicates an anagram.
'so'+'ill'+'in'+'us'='soillinus'
'soillinus' is an anagram of 'ILLUSIONS'.

'having' is the link.

Can you help me to learn more?

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

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