Train group of dolphins (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)
'group of dolphins' is the second definition.
(a group of dolphins is called a school)
(Other definitions for school that I've seen before include "Centre of learning" , "training centre" , "Sect, faction" , "a lot of swimmers" , "Place of learning" .)