Crossword Genius

Impressive though not entirely legal (5)

Ross

I believe the answer is:

awful

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'impressive' is the definition.
(thesaurus)

I cannot understand how the rest of the clue works.

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(Other definitions for awful that I've seen before include "pants" , "Atrocious, dire" , "Fearsome" , "Dreadful - spine-chilling" , "shocking" .)

I've seen this clue in The Globe and Mail.
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