Tariff applied to biological category (5)

I believe the answer is:

taxon

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'biological category' is the definition.
The definition and answer can be both groups as well as being singular nouns.
Maybe they are linked in a way I don't understand?

'tariff applied' is the wordplay.
'tariff' becomes 'tax' (I've seen this before).
'applied' becomes 'on'.
'tax'+'on'='TAXON'

'to' acts as a link.

Can you help me to learn more?

I've seen this clue in The Independent.

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