Clue: Ancient Briton carrying King Edward to stake (6)
Answer: PICKET
Explanation: Ancient Briton carrying King Edward to stake (6)

PICT ("Ancient Briton") carrying KE (King Edward)
 
*I haven't come across KE as an abbreviation for King Edward, which would surely be ER (Edward Rex). The other alternative is that K is King and E is simply Edward, but again, E is not an abbreviation for Edward, except in ER.