Thing is different in darkness (5)
I believe the answer is:
night
'darkness' is the definition.
(night is a period of darkness)
'thing is different' is the wordplay.
'is different' indicates an anagram.
'thing' with letters rearranged gives 'NIGHT'.
'in' acts as a link.
(Other definitions for night that I've seen before include "Hours of darkness" , "A Hard Day's (the Beatles)" , "Saturday -- Fever (film)" , "Dark period" , "End of the day" .)