Terrorism is more than a mere 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)

'terrorism is more than a mere' is the wordplay.
'ERROR' is hidden in amongst the letters.
But, I am uncertain how this is indicated.

Can you help me to learn more?

(Other definitions for error that I've seen before include "Mistake or mishap" , "wrong" , "Bug" , "Faulty judgment" , "Mistake or fault" .)

I've seen this clue in the Evening Standard.

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