Memory of niece's school essentially (4)
I believe the answer is:
echo
'memory' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'niece's school essentially' is the wordplay.
'niece' becomes 'ec' (I can't justify this - if you can you should believe this answer much more).
'essentially' indicates the central letters.
The central letters of 'school' are 'ho'.
'ec'+'ho'='ECHO'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for echo that I've seen before include "Mythical nymph" , "letter we hear" , "Repeating of sound" , "Reflected sound" , "Rebounding sound" .)