Impressive though not entirely legal (5)
I believe the answer is:
awful
'impressive' is the definition.
(thesaurus)
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 "pants" , "Atrocious, dire" , "Fearsome" , "Dreadful - spine-chilling" , "shocking" .)