Mistake resulting in fear, though not at first (5)
I believe the answer is:
error
'mistake' is the definition.
(I know that mistake can be written as error)
'fear though not at first' is the wordplay.
'fear' becomes 'terror' (terror is a kind of fear).
'though not at first' means to remove the first letter (the first letter is removed).
'terror' with its first letter taken away is 'ERROR'.
'resulting in' is the link.
(Other definitions for error that I've seen before include "Way of operating" , "Mistake or fault" , "Gaffe" , "Slip-up" , "wrongdoing" .)