Thing is transformed in darkness (5)
I believe the answer is:
night
'darkness' is the definition.
(night is a period of darkness)
'thing is transformed' is the wordplay.
'is transformed' indicates anagramming the letters.
'thing' is an anagram of 'NIGHT'.
'in' is the link.
(Other definitions for night that I've seen before include "'She walks in beauty, like the . . . . .' (Byron)" , "Hours of darkness" , "Time for bed" , "That's when it's darkest" , "What proverbially follows day" .)