Republic's day in the sun, almost (5)
I believe the answer is:
sudan
'sun almost' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'republic's day in' is the wordplay.
'republic's' becomes 'da' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'day' becomes 'sun' (abbreviation for Sunday).
'in' indicates putting letters inside.
'da' inserted within 'sun' is 'SUDAN'.
'the' acts as a link.
Can you help me to learn more?
(Other definitions for sudan that I've seen before include "Country south of Egypt" , "Country W. of Ethiopia" , "Country divided by the Nile" , "African nation" , "Us and the African state" .)