Crossword Genius

Not completely legal? That's terrible! (5)

Ross

I believe the answer is:

awful

I'm a little stuck... Click here to teach me more about this clue!

'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!" .)

I've seen this clue in the Sydney Morning Herald.
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