Close to endless darkness (4)
I believe the answer is:
nigh
'close to' is the definition.
(I've seen this in another clue)
'endless darkness' is the wordplay.
'endless' means to remove the last letter.
'darkness' becomes 'night' (night is a period of darkness).
'night' with its last letter taken away is 'NIGH'.
(Other definitions for nigh that I've seen before include "old touch" , "in the vicinity?" , "Close at hand" , "Near; close" , "Nearby" .)