Darkness is an odd thing (5)
I believe the answer is:
night
'darkness' is the definition.
(night is a period of darkness)
'an odd thing' is the wordplay.
'an odd' indicates an anagram.
'thing' anagrammed gives 'NIGHT'.
'is' is the link.
(Other definitions for night that I've seen before include "- School, - owl" , "From dusk until dawn" , "Period of darkness" , "For most, the time for sleep" , "period of gloom" .)