Country set in mostly established area (6)
I believe the answer is:
latvia
This could be a double definition.
'country' is the first definition.
(Latvia is a European country)
'mostly established area' is the second definition.
'Latvia' can be an answer for 'area' (Latvia is an example). I'm not sure about the 'mostly established' bit.
'set in' acts as a link.
Can you help me to learn more?
(Other definitions for latvia that I've seen before include "Baltic country" , "Northeast European country" , "Baltic republic" , "Country in northern Europe" , "Riga's country" .)