Train a swimming group (6)
I believe the answer is:
school
I believe this clue is a double definition.
'train' is the first definition.
(both can mean to teach or coach)
'a swimming group' is the second definition.
(group of fish)
(Other definitions for school that I've seen before include "Train - group of porpoises" , "See 7" , "Where 2 takes place" , "Group of porpoises - place of education" , "a lot of swimmers" .)