Thing going wrong in the dark (5)
I believe the answer is:
night
'the dark' is the definition.
(it's dark at nighttime)
'thing going wrong' is the wordplay.
'going wrong' indicates an anagram.
'thing' with letters rearranged gives 'NIGHT'.
'in' is the link.
(Other definitions for night that I've seen before include "'Red sky at . . . . ., shepherd's delight'" , "Time for bed" , "Ignorance" , "Evening" , "What proverbially follows day" .)