Fish deter swimming round Scottish island (3,6)
I believe the answer is:
red mullet
'fish' is the definition.
(red mullet is a kind of fish)
'deter swimming round scottish island' is the wordplay.
'swimming' indicates an anagram.
'round' indicates putting letters inside.
'scottish island' becomes 'mull' (Isle of Mull).
'deter' anagrammed gives 'redet'.
'redet' placed around 'mull' is 'RED MULLET'.
(Other definitions for red mullet that I've seen before include "Tasty swimmer" , "one's gathered in net" , "There's something fishy" , "Brightly coloured food fish" , "Fish -- dull meter (anag)" .)