How good, but rather frightening, the story was? (8)
I believe the answer is:
rattling
'how' is the definition.
Both the definition and answer are adverbs. Perhaps there's an association between them I don't understand?
'good but rather frightening the story was?' is the wordplay.
I cannot really understand how this works, but
'good' could be 'g' (abbreviation) and 'g' is found within the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is present in the answer.
The remaining letters 'ratlin' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for rattling that I've seen before include "Quick and energetic" , "Noisily shaking (as in good yarn)" , "Extraordinarily good, for a snake?" , "Smart" , "worrying" .)