Troop train (6)
I believe the answer is:
school
I believe this clue is a double definition.
'troop' is the first definition.
(thesaurus)
'train' is the second definition.
(I've seen this before)
(Other definitions for school that I've seen before include "Teach - fishes swimming" , "Educational facility" , "Group of sea creatures" , "Academy; group of fish" , "a lot of swimmers" .)