Nation keeps uranium in brine mixture (6)
I believe the answer is:
brunei
'nation' is the definition.
'uranium in brine mixture' is the wordplay.
'uranium' becomes 'U' (U is the chemical symbol for uranium).
'in' means one lot of letters goes inside another.
'mixture' indicates anagramming the letters.
'brine' with letters rearranged gives 'brnei'.
'u' going within 'brnei' is 'BRUNEI'.
'keeps' acts as a link.
(Other definitions for brunei that I've seen before include "Commonwealth country" , "Oil-rich sultanate of Borneo" , "Sultanate of southeast Asia" , "It's sultan is immensely rich" , "Rue bin in the sultanate" .)