Swimmer circumnavigating one's island nation (7)
I believe the answer is:
tunisia
'nation' is the definition.
(Tunisia is an example)
'swimmer circumnavigating one's island' is the wordplay.
'swimmer' becomes 'tuna' (tuna is a type of fish).
'circumnavigating' is an insertion indicator.
'one's' becomes 'i's' (Roman numeral).
'island' becomes 'i' (geographical abbreviation).
'is'+'i'='isi'
'tuna' going around 'isi' is 'TUNISIA'.
(Other definitions for tunisia that I've seen before include "Country between Algeria/Libya" , "Somewhere in North Africa" , "As Inuit visits there in North Africa" , "Republic in northwestern Africa" , "African republic on the Mediterranean coast" .)