Monet's last painting is hard work (4)
I believe the answer is:
toil
'is hard work' is the definition.
(I've seen this before)
'monet's last painting' is the wordplay.
'last' says to take the final letters.
'painting' becomes 'oil' (oil painting is a kind of painting).
The last letter of 'monet' is 't'.
't'+'oil'='TOIL'
(Other definitions for toil that I've seen before include "Manual labour" , "Work hard, labour" , "Work hard and long" , "Hard physical work" , "Drudgery" .)