The terrorist'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 terrorists' is the wordplay.
'ERROR' is hidden within the letters.
But, I'm uncertain how this is indicated.

Can you help me to learn more?

(Other definitions for error that I've seen before include "Slip-up" , "Way of operating" , "wrong" , "Howler" , "Boob" .)

I've seen this clue in The Irish Independent and the Evening Standard.

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