Swimmer studied, missing a certain style (3,6)
I believe the answer is:
red mullet
'swimmer' is the definition.
(red mullet is a kind of fish)
'studied missing a certain style' is the wordplay.
'studied' becomes 'read' (I've seen this before).
'missing' indicates named letters should be taken away.
'certain style' becomes 'mullet' (hair style).
'read' with 'a' taken away is 'red'.
'red'+'mullet'='RED MULLET'
(Other definitions for red mullet that I've seen before include "one's gathered in net" , "Tasty swimmer" , "Fish -- dull meter (anag)" , "There's something fishy" , "Brightly coloured food fish" .)