Mistake for some terrorists (5)
I believe the answer is:
error
'mistake' is the definition.
(I know that mistake can be written as error)
'some terrorists' is the wordplay.
'some' indicates the answer is hidden within the clue (some parts of the word are used).
'ERROR' is hidden within 'terrorists'.
'for' acts as a link.
(Other definitions for error that I've seen before include "Gaffe" , "Howler" , "A mistake or inaccuracy" , "Slip-up" , "Blunder, slip" .)