Dark sort of thing! (5)
I believe the answer is:
night
'dark' is the definition.
(it's dark at nighttime)
'sort of thing' is the wordplay.
'sort of' indicates anagramming the letters.
'thing' with letters rearranged gives 'NIGHT'.
(Other definitions for night that I've seen before include "- School, - owl" , "Illumination needed now" , "Day's partner" , "Period of darkness" , "Saturday -- Fever (film)" .)