Country sunk, supposedly last, ain't bothered (8)
I believe the answer is:
atlantis
'country sunk supposedly' is the definition.
(mythical lost island)
'last ain't bothered' is the wordplay.
'bothered' is an anagram indicator.
'last'+'aint'='lastaint'
'lastaint' with letters rearranged gives 'ATLANTIS'.
(Other definitions for atlantis that I've seen before include "Fabled lost island" , "Legendary 23" , "In Greek legend, beautiful island submerged in the sea" , "Drowned city of Greek myth" , "In legend, a continent under the sea" .)