Hard work, labour (4)
I believe the answer is:
toil
I believe this is a double definition.
'hard work' is the first definition.
(I've seen this in another clue)
'labour' is the second definition.
(I know that labour can be written as toil)
(Other definitions for toil that I've seen before include "Hard physical work" , "Work hard and long" , "Work hard, labour" , "Work extremely hard" , "Work really hard" .)