Fold or thickness (3)
I believe the answer is:
ply
This clue is a double definition.
'fold' is the first definition.
(I've seen this before)
'thickness' is the second definition.
(I have seen 'Thickness of yarn ' mean 'ply' so perhaps 'thickness' could also mean 'ply')
(Other definitions for ply that I've seen before include "Work steadily as at one's trade" , "Layer of folded material" , "Laminate thickness" , "'Persist in offering, liquor say (3)'" , "Work steadily at the strand or layer" .)