Astonishing speed? (3)

I believe the answer is:

ton

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

'speed?' is the definition.
(I've seen this before)

'astonishing' is the wordplay.
'TON' is hidden in amongst the letters.
But, I'm not sure how this is indicated.

Can you help me to learn more?

(Other definitions for ton that I've seen before include "Fashionable" , "See 25" , "20 cwt" , "100 mph, runs, etc" , "Two thousand, two hundred and forty lbs avoirdupois" .)

I've seen this clue in the Evening Standard.

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