The mistake of terrorism (5)
I believe the answer is:
error
'the mistake' is the definition.
('error' can be a synonym of 'mistake')
'of terrorism' is the wordplay.
'of' indicates the answer is hidden within the clue.
'ERROR' is hidden within 'terrorism'.
(Other definitions for error that I've seen before include "Boo-boo" , "Miscarriage of justice" , "Solecism" , "Fallacy, mistake" , "Slip-up" .)