A hugely experienced topiarist? That's Serpent! (10)

I believe the answer is:

bushmaster

I'm a little stuck... Click here to teach me more about this clue!

This clue could be a double definition.

'a hugely experienced topiarist?' is the first definition.
The definition suggests a singular noun which matches the answer.

'serpent' is the second definition.
Both the answer and definition are singular nouns.
Maybe there's a link between them I don't understand?

'that's' is the link.

Can you help me to learn more?

(Another definition for bushmaster that I've seen is " Pit viper; sham brutes (anag.)".)

I've seen this clue in The Independent.

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