Frightening word, repeating error (3-3)
I believe the answer is:
boo-boo
'error' is the definition.
(I know that boo-boo is a type of error)
'frightening word repeating' is the wordplay.
'frightening word' becomes 'boo' (word that may be said to surprise someone).
'repeating' shows that letters should be duplicated.
'boo' duplicated is 'BOO-BOO'.
(Other definitions for boo-boo that I've seen before include "Mistake" , "Clanger" .)