Baker's agent in unknown quarter (5)
I believe the answer is:
yeast
'baker's agent' is the definition.
'yeast' can be an answer for 'agent' (thesaurus). I'm not certain of the 'baker's' bit.
'unknown quarter' is the wordplay.
'unknown' becomes 'y' (algebra).
'quarter' becomes 'east' (I've seen this in other clues).
'y'+'east'='YEAST'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for yeast that I've seen before include "It makes dough rise" , "Beer froth agent" , "Fungi used in bakery" , "Dough raising agent" , "Fungus used in fermenting" .)