Teach swimming group (6)
I believe the answer is:
school
This clue is a double definition.
'teach' is the first definition.
(I've seen this in another clue)
'swimming group' is the second definition.
(group of fish)
(Other definitions for school that I've seen before include "Instruct" , "Educate, train" , "For the education of fish" , "Centre of learning" , "Forms" .)