Unwell? That's awful (3)
I believe the answer is:
bad
I believe this clue is a double definition.
'unwell?' is the first definition.
(thesaurus)
'awful' is the second definition.
(similar in meaning)
'that's' acts as a link.
(Other definitions for bad that I've seen before include "Spoiled or evil" , "(Of food) decayed" , "not healthy" , "Stinking" , "Wicked" .)