Toiling away (4,2,4)
(Other definitions for atit that I've seen before include "what birds (and bees) do?" , "up to no good?" , "in Congress" .)
'toiling away' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for at work that I've seen before include "busy" , "Active" , "Engaged in one's duties" .)