Thickness (7)
'thickness' is the definition.
(I have seen 'Thickness of yarn ' mean 'ply' so perhaps 'thickness' could also mean 'ply')
This is all the clue.
(Other definitions for ply that I've seen before include "A layer to work at diligently." , "Use or wield, tool or weapon" , "Strand, of wool or wood, say" , "Layer of folded material" , "'Persist in offering, liquor say (3)'" .)
'thickness' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for density that I've seen before include "showing obtuseness" , "there's stupidity for you!" , "Bulk" , "Mass per volume" , "can be relative" .)