Hide the mad king outside! (7)
I believe the answer is:
leather
'hide' is the definition.
(an animal hide)
'the mad king outside' is the wordplay.
'mad king' becomes 'lear' (King Lear).
'outside' means one lot of letters goes inside another.
'the' put within 'lear' is 'LEATHER'.
(Other definitions for leather that I've seen before include "Beat" , "Material made from animal skin" , "Morocco, perhaps" , "Dressed hide" , "Tanned animal hide" .)