Former schoolmates set about ousting King and Bar (8)

I believe the answer is:
obstacle
'bar' is the definition.
(I've seen this before)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for obstacle that I've seen before include "bar" , "Maybe let" , "It obstructs or hinders" , "Progress blocker" , "hold-up" .)
