Animal that's an insect (7)

I believe the answer is:

beastie

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'an insect' is the definition.
Both the definition and answer are singular nouns.
Perhaps you can see a link between them that I don't see?

'animal that's' is the wordplay.
'animal' becomes 'beast' (synonyms).
'that's' becomes 'ie'.
'beast'+'ie'='BEASTIE'

Can you help me to learn more?

(Another definition for beastie that I've seen is " Rabbie Burns's wee mouse".)

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

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