Nursery periodically purged a significant number of fish (9)
I believe the answer is:
preschool
'nursery' is the definition.
(I've seen this in another clue)
'periodically purged a significant number of fish' is the wordplay.
'periodically' means one should take alternating letters (letters must be selected 'every so often').
'a significant number of fish' becomes 'school' (I can't explain this - if you can you should believe this answer much more).
The alternate letters of 'purged' are 'pre'.
'pre'+'school'='PRESCHOOL'
Can you help me to learn more?
(Other definitions for preschool that I've seen before include "before education" , "provided for kiddies?" , "early years" , "Kindergarten" .)