Not completely legal? That's terrible! (5)
I believe the answer is:
awful
'terrible' is the definition.
(I know that terrible can be written as awful)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for awful that I've seen before include "Horrible" , "dire result" , "Dreadful, nasty" , "Exceptionally bad" , "gruesome!" .)