Darkness is terrible thing (5)
I believe the answer is:
night
'darkness' is the definition.
(night is a period of darkness)
'terrible thing' is the wordplay.
'terrible' is an anagram indicator.
'thing' with letters rearranged gives 'NIGHT'.
'is' acts as a link.
(Other definitions for night that I've seen before include "Ignorance" , "A Hard Day's (the Beatles)" , "Illumination needed now" , "Time between days" , "Hours of darkness" .)