Laborious task (4)
I believe the answer is:
slog
'laborious task' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for slog that I've seen before include "Work (or walk) involving great effort" , "Very tiring work" , "Exhausting task" , "almighty wallop" , "Work doggedly" .)