Fold, thickness (3)
I believe the answer is:
ply
I believe this is a double definition.
'fold' is the first definition.
(I've seen this before)
'thickness' is the second definition.
(I've seen this in another clue)
(Other definitions for ply that I've seen before include "This wood is laminated" , "Work steadily at, say trade" , "Use or wield, tool or weapon" , "'Persist in offering, liquor say (3)'" , "A layer to work at diligently." .)