Launch requisition on where in France to find unknown sauce (4)
I believe the answer is:
roux
'sauce' is the definition.
(I know that roux is a sauce)
'launch requisition on where in france to find unknown' is the wordplay.
'launch requisition on' becomes 'r' (I can't explain this - if you can you should believe this answer much more).
'where in france' becomes 'ou' ('where' in French).
'to find' says to put letters next to each other.
'unknown' becomes 'x' (algebra).
'r'+'ou'+'x'='ROUX'
Can you help me to learn more?
(Other definitions for roux that I've seen before include "Mixture of butter and flour used for sauces" , "sauce mix" , "Mixture of fat and flour used as a base for sauces" , "Cook's thickening" , "Thickener for sauces" .)