On reflection, observe educational establishment (4)
I believe the answer is:
eton
'educational establishment' is the definition.
The definition suggests a singular noun which matches the answer.
'on reflection observe' is the wordplay.
'on reflection' says the letters should be written backwards.
'observe' becomes 'note' (synonyms).
'note' backwards is 'ETON'.
Can you help me to learn more?
(Other definitions for eton that I've seen before include "Public school near Windsor" , "Famous English public school" , "Public school (sounds like 20)" , "Public school founded by Henry VI" , "academic institution" .)