See simple-minded Nationalist ousting Liberal (7)
I believe the answer is:
witness
'see' is the definition.
(to witness something is to see it happen)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for witness that I've seen before include "Observer" , "See (an event or crime)" , "Bystander" , "One sees and reports" , "Evidence, proof" .)