Looking down on king, old hat (8)
I believe the answer is:
sombrero
'hat' is the definition.
(I know that sombrero is a type of hat)
'looking down on king old' is the wordplay.
'looking down' becomes 'sombre' (sombre can mean looking miserable or down).
'on' is a charade indicator (letters next to each other).
'king' becomes 'R' (abbreviation for rex, king in Latin).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'sombre'+'r'+'o'='SOMBRERO'
(Other definitions for sombrero that I've seen before include "Straw hat with very wide brim from Mexico" , "Wide-brimmed Mexican hat" , "Large Mexican sun-hat" , "Covering up" , "headgear in parts of US" .)