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.
(synonyms)

'the terrorists' is the wordplay.
'ERROR' is hidden within the letters.
But, I am uncertain how the hidden word 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 Evening Standard and The Irish Independent.

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