Sauce base (4)
I believe the answer is:
roux
'sauce base' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for roux that I've seen before include "Fat and flour mixture, cooked and used to make sauces" , "Mixture of flour and fat for cooking" , "Thickener for sauces" , "Fat, flour sauce base" , "Mixture of butter and flour used for sauces" .)