Hard work — work hard (4)
I believe the answer is:
toil
I believe this clue is a double definition.
'hard work' is the first definition.
(I've seen this before)
'work hard' is the second definition.
(to toil is to do strenuous work)
(Other definitions for toil that I've seen before include "Work incessantly" , "Work hard and long" , "Drudgery" , "Hard physical work" , "Manual labour" .)