Teach group of swimmers (6)
I believe the answer is:
school
I believe this is a double definition.
'teach' is the first definition.
(I've seen this in another clue)
'group of swimmers' is the second definition.
(school can mean a group of fish)
(Other definitions for school that I've seen before include "group of dolphins, for example" , "Teach; group of fish" , "Teaching establishment" , "these like-minded people could follow" , "College" .)