The terrorist's blunder (5)

I believe the answer is:

error

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

'blunder' is the definition.
(I've seen this before)

'the terrorist's' is the wordplay.
'ERROR' is hidden within the letters.
However, I am unsure how this is indicated.

Can you help me to learn more?

(Other definitions for error that I've seen before include "wrong" , "Howler" , "Bloomer, blunder" , "Slip, mistake" , "Slip-up" .)

I've seen this clue in the Evening Standard.

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