Shakespearean king pens the material (7)
I believe the answer is:
leather
'material' is the definition.
(I know that leather is a type of clothing material)
'shakespearean king pens the' is the wordplay.
'shakespearean king' becomes 'Lear' (King Lear).
'pens' indicates putting letters inside.
'lear' enclosing 'the' is 'LEATHER'.
(Other definitions for leather that I've seen before include "Tanned animal hide" , "durable material" , "Material made from animal skin" , "kid, say" , "Treated animal skin" .)