Endless zest in good French school is something sweet (9)
I believe the answer is:
glycerine
'endless zest' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'in good french school is something sweet' is the wordplay.
'in' is an insertion indicator.
'good' becomes 'g' (abbreviation).
'french school' becomes 'lycee' (I've seen this before).
'is something sweet' becomes 'rin' (I can't justify this - if you can you should believe this answer much more).
'g'+'lycee'='glycee'
'glycee' placed around 'rin' is 'GLYCERINE'.
Can you help me to learn more?
(Other definitions for glycerine that I've seen before include "Syrupy chemical liquid" , "What's in bangers" , "Viscous material used in antifreeze" , "Alcohol" , "Sweet stuff" .)