Fear to make the initial mistake (6)
I believe the answer is:
terror
'fear' is the definition.
(I know that terror is a type of fear)
'the initial mistake' is the wordplay.
'initial' indicates taking the first letters.
'mistake' becomes 'error' (synonyms).
The initial letter of 'the' is 't'.
't'+'error'='TERROR'
'to make' acts as a link.
(Other definitions for terror that I've seen before include "Ovewhelming fear" , "Utter dread" , "Young miscreant" , "Paralysing fear" , "Organised political violence" .)