Work sheet (3)
I believe the answer is:
ply
I believe this clue is a double definition.
'work' is the first definition.
(to ply a trade is to work at it)
'sheet' is the second definition.
(ply is a kind of sheet)
(Other definitions for ply that I've seen before include "Use or wield, tool or weapon" , "Fold; thickness" , "Travel a route regularly" , "Laminate thickness" , "Work steadily at the strand or layer" .)