Yachtsman catching large fish (7)
I believe the answer is:
bloater
'fish' is the definition.
(bloater is a kind of fish)
'yachtsman catching large' is the wordplay.
'yachtsman' becomes 'boater' (I've seen this in another clue).
'catching' is an insertion indicator.
'large' becomes 'L' (eg in clothes sizes).
'boater' going around 'l' is 'BLOATER'.
(Other definitions for bloater that I've seen before include "that will more than fill one up" , "Buckling" , "Something fishy" , "Fish" , "Herring salted and smoked" .)