Swimmers' academy? (6)
I believe the answer is:
school
I believe this clue is a double definition.
'swimmers'' is the first definition.
(group of fish)
'academy?' is the second definition.
(academy is a kind of school)
(Other definitions for school that I've seen before include "Train - group of whales" , "Where one teaches" , "Educational facility" , "lots of swimmers" , "A number of swimmers joined" .)