Near heart of total darkness (5)
I believe the answer is:
night
'darkness' is the definition.
(night is a kind of darkness)
'near heart of total' is the wordplay.
'near' becomes 'nigh' ('nigh' can be a synonym of 'near'**).
'heart of' says to take the centre.
The middle of 'total' is 't'.
'nigh'+'t'='NIGHT'
(Other definitions for night that I've seen before include "See 1 Across" , "death" , "Time for bed" , "8d [DARKNESS]" , "Dark hours" .)