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 present in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found in the answer.
The remaining letters 'ratlin' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
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" .)