Close to unending darkness (4)
I believe the answer is:
nigh
'close to' is the definition.
(thesaurus)
'unending darkness' is the wordplay.
'unending' means to remove the last letter (the word doesn't have its end letter).
'darkness' becomes 'night' (night is a period of darkness).
'night' with its final letter removed is 'NIGH'.
(Other definitions for nigh that I've seen before include "Adjacent" , "Near in time or place" , "Close at hand" , "Near; close" , "Almost (arch.)" .)