He said sesame and lettuce in buns are "burger amateur's toppings" (3,4)
I believe the answer is:
ali baba
'he said sesame and lettuce in buns are burger amateur's toppings' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'he said sesame' is the wordplay.
'he' becomes 'Ali'.
'said sesame' becomes 'baba' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ali'+'baba'='ALI BABA'
Can you help me to learn more?
(Other definitions for ali baba that I've seen before include "Fabled woodcutter" , "'...... and the Forty Thieves (3,4)'" , "Chopper" , "fabulous character" , "Hero of Arabic story" .)