Country is one investing in fish (7)
I believe the answer is:
tunisia
'country' is the definition.
(I know that Tunisia is a country)
'is one investing in fish' is the wordplay.
'one' becomes 'i' (Roman numeral).
'investing in' means one lot of letters goes inside another.
'fish' becomes 'tuna' (tuna is a type of fish).
'is'+'i'='isi'
'isi' inserted into 'tuna' is 'TUNISIA'.
(Other definitions for tunisia that I've seen before include "North African republic" , "Somewhere in North Africa" , "Country where Carthage is found" , "Republic in northwestern Africa" , "African republic on the Mediterranean coast" .)