Thing moving in darkness (5)
I believe the answer is:
night
'darkness' is the definition.
(night is a period of darkness)
'thing moving' is the wordplay.
'moving' indicates anagramming the letters (letters are moved around).
'thing' with letters rearranged gives 'NIGHT'.
'in' acts as a link.
(Other definitions for night that I've seen before include "Evening" , "Dark period" , "Day's partner" , "Sunset to sunrise" , "Illumination needed now" .)