Great fear triggered initially by mistake (6)
I believe the answer is:
terror
'great fear' is the definition.
(I've seen this before)
'triggered initially by mistake' is the wordplay.
'initially' indicates taking the first letters.
'by' means one lot of letters go next to another.
'mistake' becomes 'error' (synonyms).
The first letter of 'triggered' is 't'.
't'+'error'='TERROR'
(Other definitions for terror that I've seen before include "Extreme or intense fear" , "Utter dread" , "Fear - mischievous child" , "Troublesome child" , "rogue" .)