Prime collection of fish (6)
I believe the answer is:
school
'prime' is the definition.
(thesaurus)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for school that I've seen before include "group of dolphins, for example" , "Institute of education" , "For the education of fish" , "A number of fish" , "Group of sea creatures" .)