Coach or train (6)
I believe the answer is:
school
This clue is a double definition.
'coach' is the first definition.
(both can mean to teach or train)
'train' is the second definition.
(both can mean to teach or coach)
'or' is the link.
(Other definitions for school that I've seen before include "some bass?" , "A number of swimmers joined" , "College" , "Sect, faction" , "A number of fish" .)