Endless darkness approaching (4)
I believe the answer is:
nigh
'approaching' is the definition.
(thesaurus)
'endless darkness' is the wordplay.
'endless' means to remove the last letter.
'darkness' becomes 'night' (night is a kind of darkness).
'night' with its last letter taken away is 'NIGH'.
(Other definitions for nigh that I've seen before include "Nearby" , "Impending" , "old touch" , "Old word for near, as the end may be" , "Near, as in archaic usage" .)