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' says the answer is hidden in the clue (some parts of the word are used).
'ERROR' is hidden in the letters of '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" .)