Part of a part of a family of US gnus, be excited! (8)

I believe the answer is:

subgenus

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'part of a part of a family' is the definition.
I can't judge whether this defines the answer.

'us gnus be excited' is the wordplay.
'excited' is an anagram indicator.
'us'+'gnus'+'be'='usgnusbe'
'usgnusbe' with letters rearranged gives 'SUBGENUS'.

'of' is the link.

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I've seen this clue in the Sydney Morning Herald.

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