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 in amongst the letters.
But, I'm unsure 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 Evening Standard and The Irish Independent.

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