Great fear is more than a mistake (6)
I believe the answer is:
terror
'great fear' is the definition.
(I've seen this in another clue)
'more than a mistake' is the wordplay.
'more than' becomes 't' (I can't explain this - if you can you should believe this answer much more).
'a mistake' becomes 'error' ('error' can be a synonym of 'mistake').
't'+'error'='TERROR'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for terror that I've seen before include "Troublesome child" , "Utter dread" , "Troublesome kid" , "Overwhelming feeling of fear" , "consternation" .)