Fish endlessly on island, a Mediterranean one (8)
I believe the answer is:
sardinia
'mediterranean one' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'fish endlessly on island a' is the wordplay.
'fish' becomes 'sardine' (sardine is a type of fish).
'endlessly' means to remove the last letter.
'on island' becomes 'i' (I can't explain this - if you can you should believe this answer much more).
'sardine' with its final letter removed is 'sardin'.
'sardin'+'i'+'a'='SARDINIA'
(Other definitions for sardinia that I've seen before include "Large Italian island, capital Cagliari" , "Island south of Corsica" , "Corsican neighbour" , "Adrian is seen in Mediterranean island" , "Second-largest Mediterranean island, after Sicily" .)