Terrible thing, darkness (5)
I believe the answer is:
night
'darkness' is the definition.
(night is a kind of darkness)
'terrible thing' is the wordplay.
'terrible' is an anagram indicator.
'thing' anagrammed gives 'NIGHT'.
(Other definitions for night that I've seen before include "That's when it's darkest" , "What proverbially follows day" , "For most, the time for sleep" , "Illumination needed now" , "late" .)