Last unnecessarily long mythical creature (6)

I believe the answer is:

dragon

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

'mythical creature' is the definition.
(dragon is a kind of mythical creature)

'last unnecessarily long' is the wordplay.
I cannot really see how this works, but
'last' could be 'go' (synonyms) and 'go' is found in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for dragon that I've seen before include "[TERMAGANT]" , "Mythical beast or lizard" , "Seventeen [HARRIDAN]" , "Fire-breathing beast" , "particularly intimidating woman" .)

I've seen this clue in The Telegraph.

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