The initial mistake causes panic (6)
I believe the answer is:
terror
'panic' is the definition.
(I've seen this before)
'the initial mistake' is the wordplay.
'initial' indicates taking the first letters.
'mistake' becomes 'error' ('error' can be a synonym of 'mistake').
The first letter of 'the' is 't'.
't'+'error'='TERROR'
'causes' acts as a link.
(Other definitions for terror that I've seen before include "rogue" , "Panicky fright" , "Troublesome child" , "Fear - mischievous child" , "Utter dread" .)