Milling around is ancient reptile (8)

I believe the answer is:

dinosaur

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

'reptile' is the definition.
(I know that dinosaur is a type of dinosaur)

'milling around is ancient' is the wordplay.
'around is' can be anagrammed to 'DINOSAUR'.
Though, I'm not clear how this is indicated.

Can you help me to learn more?

(Other definitions for dinosaur that I've seen before include "Reptile is around, not really" , "Large extinct creature" , "Fogey" , "Huge prehistoric monster" , "Old-fashioned type" .)

I've seen this clue in The Sun.

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