Death is a grim one (6)
I believe the answer is:
reaper
'death is a grim one' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for reaper that I've seen before include "Death is described as a grim one" , "that you don't want to meet if it's grim" , "One harvesting crop" , "One gathers" , "Death is known as a grim one" .)