Horrified as a good speed is cut (6)
I believe the answer is:
aghast
'horrified' is the definition.
(I know that horrified can be written as aghast)
'as a good speed is cut' is the wordplay.
I cannot quite understand how this works, but
'as' is found in the answer.
'a' is present in the answer.
'good' could be 'g' (abbreviation) and 'g' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for aghast that I've seen before include "Filled with terror" , "Horror-struck" , "Horrified, appalled" , "What's carried in bag has teacher" , "Thunderstruck" .)