Tissue thickness (3)
I believe the answer is:
ply
'tissue thickness' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ply that I've seen before include "A layer to work at diligently." , "Work steadily at, say trade" , "Fold; thickness" , "drama one's missed" , "This wood is laminated" .)