Republic locks king out - ‘It's over! (5)
I believe the answer is:
haiti
'republic' is the definition.
(I know that Haiti is a republic)
'locks king out 'it's over' is the wordplay.
'locks king out' becomes 'hai' (I can't explain this - if you can you should give a lot more credence to this answer).
'over' says the letters should be written in reverse.
'it' back-to-front is 'ti'.
'hai'+'ti'='HAITI'
Can you help me to learn more?
(Other definitions for haiti that I've seen before include "Caribbean nation" , "earthquake-hit area" , "one in the Caribbean" , "Country in W Indies, capital Port-au-Prince" , "Western part of Hispaniola" .)