King taking speed for depression (6)
I believe the answer is:
crater
'for depression' is the definition.
(crater is a kind of depression)
'king taking speed' is the wordplay.
'king' becomes 'cr' (short for Carolus Rex, King Charles in Latin).
'taking' is an insertion indicator.
'speed' becomes 'rate' (the speed at which something happens).
'cr' enclosing 'rate' is 'CRATER'.
(Other definitions for crater that I've seen before include "Bowl-shaped opening" , "Volcanic depresssion" , "Volcanic depression" , "Hole (on the moon?)" , "Volcano's mouth" .)