A number of swimmers train (6)
I believe the answer is:
school
This clue is a double definition.
'a number of swimmers' is the first definition.
(I've seen this before)
'train' is the second definition.
(both can mean to teach or coach)
(Other definitions for school that I've seen before include "training centre" , "Educational establishment" , "Educational facility" , "company that's all at sea" , "College" .)