First time terrorist's abandoned wrong opinion (5)
I believe the answer is:
Here is my best explanation:
'first time terrorist's abandoned' is the definition.
I can't tell whether this definition defines the answer.
'wrong opinion' is the subsidiary indication.
'wrong' becomes 'err'.
'opinion' becomes 'or' (I can't explain this - if you can you should believe this answer much more).
(Other definitions for error that I've seen before include "Miscarriage of justice", "Faulty judgement", "Howler", "Mistake from terrorist", "boob", "Blunder or mistake", "It's just a mistake", "Boo-boo", "Gaffe" and "wrong".)