Train swimmers in group (6)
I believe the answer is:
school
This clue is a triple definition!
'train' is the first definition.
(I've seen this before)
'swimmers' is the second definition.
(group of fish)
'in' is a link.
'group' is the third definition.
(school is a kind of group)
(Other definitions for school that I've seen before include "training centre" , "College" , "Educational institution" , "Teach; group of fish" , "Teaching establishment" .)