Darkness close: Satan in the middle (5)
I believe the answer is:
night
'darkness' is the definition.
(night is a period of darkness)
'close satan in the middle' is the wordplay.
'close' becomes 'nigh' (similar in meaning).
'in the middle' indicates the central letters.
The middle letter of 'satan' is 't'.
'nigh'+'t'='NIGHT'
(Other definitions for night that I've seen before include "- School, - owl" , "A Hard Day's (the Beatles)" , "Period of darkness" , "Day's partner" , "Illumination needed now" .)