Fear of starting trouble by mistake (6)
I believe the answer is:
terror
'fear of starting' is the definition.
'terror' can be an answer for 'fear' (terror is a kind of fear). I am not certain of the 'of starting' bit.
'trouble by mistake' is the wordplay.
'trouble' becomes 't' (this could be a standard abbreviation I've not previously seen).
'by' means one lot of letters go next to another.
'mistake' becomes 'error' (synonyms).
't'+'error'='TERROR'
Can you help me to learn more?
(Other definitions for terror that I've seen before include "State of great fright" , "(What causes) panic" , "Organised political violence" , "State of extreme fear" , "Extreme or intense fear" .)