Extremely bad or serious (8)
I believe the answer is:
terrible
I believe this clue is a double definition.
'extremely bad' is the first definition.
(I've seen this before)
'serious' is the second definition.
(thesaurus)
(Other definitions for terrible that I've seen before include "Awful, dreadful" , "Appalling" , "Awful, dire" , "Atrocious" , "Horrifying" .)