Soviet film about Scottish island swimmer (3,6)
I believe the answer is:
red mullet
'swimmer' is the definition.
(red mullet is a kind of fish)
'soviet film about scottish island' is the wordplay.
'soviet' becomes 'red' (red can informally mean communist).
'film' becomes 'ET' (E.T. the Extra-Terrestrial).
'about' is a charade indicator (letters next to each other).
'scottish island' becomes 'mull' (Isle of Mull).
'et' after 'mull' is 'mullet'.
'red'+'mullet'='RED MULLET'
(Other definitions for red mullet that I've seen before include "There's something fishy" , "Tasty swimmer" , "Brightly coloured food fish" , "one's gathered in net" , "Fish -- dull meter (anag)" .)