Mistake made by 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 of the parts of the word are used).
'ERROR' is hidden within 'terrorists'.
'made by' acts as a link.
(Other definitions for error that I've seen before include "heresy" , "Faux pas" , "Slip, mistake" , "Mistake, lapse" , "Miscalculation" .)