Tissue layer (3)
'tissue layer' 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 "Thickness of yarn and wood" , "Fold; thickness" , "Travel a route regularly" , "Strand, of wool or wood, say" , "Use or wield, tool or weapon" .)
'tissue layer' is the definition.
(synonyms)
This is all the clue.
(Other definitions for membrane that I've seen before include "Thin sheet that surrounds body parts" , "Thin, pliable sheet of living tissue" , "Thin flexible film" , "Cell lining" , "Thin skin-like tissue lining organs" .)