How nasty rattles can frighten one (7)

I believe the answer is:

startle

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

'frighten one' is the definition.
'startle' can be an answer for 'frighten' (I've seen this before). I'm not certain of the 'one' bit.

'how nasty rattles' is the wordplay.
'rattles' can be anagrammed to 'STARTLE'.
But, I'm not clear how the anagram is indicated.

'can' acts as a link.

Can you help me to learn more?

(Other definitions for startle that I've seen before include "Cause to feel sudden alarm" , "Cause shock to" , "Astonish" , "Surprise with fright" , "Take by surprise" .)

I've seen this clue in the Evening Standard.

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