Panic is the first mistake (6)
I believe the answer is:
terror
'panic' is the definition.
(I know that panic can be written as terror)
'the first mistake' is the wordplay.
'the first' becomes 't' (1st letter of 'the').
'mistake' becomes 'error' (synonyms).
't'+'error'='TERROR'
'is' is the link.
(Other definitions for terror that I've seen before include "Extreme or intense fear" , "Mischief-maker; dread" , "Utter dread" , "(What causes) panic" , "Extreme fright or dread" .)