Endless hard work? Smile! (4)
I believe the answer is:
grin
'smile' is the definition.
(grinning is a kind of smiling)
'endless hard work?' is the wordplay.
'endless' means to remove the last letter.
'hard work?' becomes 'grind' (I've seen this before).
'grind' with its last letter taken off is 'GRIN'.
(Other definitions for grin that I've seen before include "Smile - all that could be seen of the vanishing Cheshire Cat" , "Facial expression showing amusement or pleasure" , "Give a broad smile" , "Smirk" , "Facial expression, usually of pleasure" .)