Tediously persist, in a word, as monster (6)

I believe the answer is:

dragon

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

'monster' is the definition.
(I know that dragon is a type of mythical creature)

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for dragon that I've seen before include "Mythical fire-breathing creature" , "Mythical fire-breathing animal" , "Teutonic mythological creature" , "TV investor" , "Fire-breathing creature slain by St George" .)

I've seen this clue in The Guardian.

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