Hard work finishing early? Smile (4)
I believe the answer is:
grin
'smile' is the definition.
(grinning is a kind of smiling)
'hard work finishing early?' is the wordplay.
'hard work' becomes 'grind' (I've seen this before).
'finishing early?' means to remove the last letter.
'grind' with its final letter taken off is 'GRIN'.
(Other definitions for grin that I've seen before include "Give a broad smile" , "Ring (anag.)" , "Look happy" , "Facial expression showing amusement or pleasure" , "Wide smile" .)