Restrained about blunder if frightened (9)
I believe the answer is:
terrified
'frightened' is the definition.
(terrifying is a kind of frightening)
'restrained about blunder if' is the wordplay.
I cannot really see how this works, but
'blunder' could be 'err' (I've seen this before) and 'err' is present in the answer.
'if' is found in the answer.
'restrained' could be 'tied' (tying is a kind of restraining) and 'tied' is found in the leftover letters.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for terrified that I've seen before include "Extremely frightened" , "Greatly frightened" , "Scared stiff" , "Panic-stricken - dire refit (anag)" .)