Toil endlessly to produce smile (4)
I believe the answer is:
grin
'smile' is the definition.
(grinning is a kind of smiling)
'toil endlessly' is the wordplay.
'toil' becomes 'grind' (grind is a kind of toil).
'endlessly' means to remove the last letter.
'grind' with its last letter taken away is 'GRIN'.
'to produce' is the link.
(Other definitions for grin that I've seen before include "Toothy smile" , "Smile - all that could be seen of the vanishing Cheshire Cat" , "11 smile" , "Smile broadly showing teeth" , "A broad smile showing the teeth" .)