Disturbed islander losing large fish (7)
I believe the answer is:
sardine
'fish' is the definition.
(sardine is a kind of fish)
'disturbed islander losing large' is the wordplay.
'disturbed' indicates an anagram.
'losing' is a deletion indicator.
'large' becomes 'l'.
'islander' with 'l' taken away is 'isander'.
'isander' is an anagram of 'SARDINE'.
(Other definitions for sardine that I've seen before include "Rinsed a (anag) - fish" , "Fish (often tinned)" , "Small fish, usually canned" , "Young pilchard" , "A diner's small fish" .)