To fade away outright is terrible! (4)
I believe the answer is:
dire
'is terrible' is the definition.
(synonyms)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for dire that I've seen before include "Fearful" , "Ominous; very bad" , "to speak (13) [FRENCH]" , "Desperate, horrendous" , "Extremely serious or urgent" .)