Really bad (8)
(Other definitions for dreadful that I've seen before include "Dire, alarming" , "Appalling, awful" , "Terrible" , "Extremely bad or disagreeable" , "Horrible, shocking" .)
'really bad' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for terrible that I've seen before include "Frightful" , "What precedes twos < and follows child in France" , "Horrifying" , "Extremely bad" , "Like tsar Ivan IV" .)