Crossword Genius

How one knows at once someone's surprised? (4,3,5)

Ross

I believe the answer is:

from the start

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

'surprised?' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.

'how one knows at once someone's' is the wordplay.
I cannot quite see how this works, but
'one' could be 'the' (both can introduce a definite example of something) and 'the' is present in the answer.
'someone' could be 'tart' (tart is a kind of someone) and 'tart' is found within the answer.
This explanation may well be incorrect...

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I've seen this clue in The Globe and Mail.
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