Darkness is a strange thing (5)
I believe the answer is:
night
'darkness' is the definition.
(night is a period of darkness)
'strange thing' is the wordplay.
'strange' indicates an anagram.
'thing' is an anagram of 'NIGHT'.
'is a' acts as a link.
(Other definitions for night that I've seen before include "'She walks in beauty, like the . . . . .' (Byron)" , "Time without 10" , "End of the day" , "Time for bed" , "Time for turning" .)