The old way to secure a baker's requisite (5)
I believe the answer is:
yeast
'baker's requisite' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'the old way to secure a' is the wordplay.
'the old' becomes 'ye' (resembles a historical spelling of 'the').
'way' becomes 'st' (abbreviation for street).
'to secure' is an insertion indicator (inserted letters are secured inside).
'ye'+'st'='yest'
'yest' going around 'a' is 'YEAST'.
Can you help me to learn more?
(Other definitions for yeast that I've seen before include "Fungus used to ferment sugar" , "Fungus used in brewing and baking" , "A raising agent in baking" , "Dough-raising agent" , "fermenter" .)