Tragic king pens the material (7)
I believe the answer is:
leather
'material' is the definition.
(I've seen this before)
'tragic king pens the' is the wordplay.
'tragic king' becomes 'lear' (I've seen this before).
'pens' indicates putting letters inside.
'lear' enclosing 'the' is 'LEATHER'.
(Other definitions for leather that I've seen before include "Processed animal skin" , "Polishing cloth" , "Beat" , "Tanned animal skin" , "Thrash" .)