A 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)

'a terrorist's' is the wordplay.
'ERROR' is hidden within the letters.
But, I'm uncertain how the hidden word is indicated.

Can you help me to learn more?

(Other definitions for error that I've seen before include "wrongdoing" , "Miscalculation" , "Slip, mistake" , "Fallacy, mistake" , "Gaffe" .)

I've seen this clue in The Irish Independent.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.