Drive feudal changes? That's very bad (8)
I believe the answer is:
dreadful
'very bad' is the definition.
(I've seen this before)
'drive feudal changes?' is the wordplay.
'drive' becomes 'dr' (I've seen this in other clues).
'changes?' indicates anagramming the letters (I've seen 'change' mean this).
'feudal' anagrammed gives 'eadful'.
'dr'+'eadful'='DREADFUL'
'that's' acts as a link.
(Other definitions for dreadful that I've seen before include "Terrible" , "Dire, alarming" , "Atrocious, awful" , "Informally very bad, perhaps the weather" , "Horrible, shocking" .)