Not far from endless darkness (4)
I believe the answer is:
nigh
'not far from' is the definition.
(thesaurus)
'endless darkness' is the wordplay.
'endless' means to remove the last letter.
'darkness' becomes 'night' (night is a period of darkness).
'night' with its final letter taken away is 'NIGH'.
(Other definitions for nigh that I've seen before include "Near, as in archaic usage" , "All but one" , "'Almost, near (4)'" , "in the vicinity?" , "Just round the corner" .)