Fear mistake after beginning of trial (6)
I believe the answer is:
terror
'fear' is the definition.
(I know that terror is a type of fear)
'mistake after beginning of trial' is the wordplay.
'mistake' becomes 'error' (synonyms).
'after' means one lot of letters go next to another.
'beginning of trial' becomes 't' (I can't explain this - if you can you should believe this answer much more).
'error' after 't' is 'TERROR'.
Can you help me to learn more?
(Other definitions for terror that I've seen before include "Troublesome child" , "Organised political violence" , "Young miscreant" , "(What causes) panic" , "State of extreme fear" .)