Leader briefly annexing capital of large country (5)
I believe the answer is:
chile
'large country' is the definition.
'chile' can be an answer for 'country' (Chile is a country). I am not sure about the 'large' bit.
'leader briefly annexing capital' is the wordplay.
'leader' becomes 'chief' (chief is a kind of leader).
'briefly' means to remove the last letter.
'annexing' indicates putting letters inside.
'capital' becomes 'l' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'chief' with its final letter taken away is 'chie'.
'chie' enclosing 'l' is 'CHILE'.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for chile that I've seen before include "Most southerly American country" , "South American country" , "Long narrow country" , "S. American nation" , "Long, thin country" .)