The terroriser's mistake (5)

I believe the answer is:

error

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

'mistake' is the definition.
(I know that mistake can be written as error)

'the terroriser's' is the wordplay.
'ERROR' is hidden within the letters.
However, I'm not sure how this is indicated.

Can you help me to learn more?

(Other definitions for error that I've seen before include "Gaffe" , "Way of operating" , "Bug" , "wrongdoing" , "Blunder or mistake" .)

I've seen this clue in the Evening Standard.

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