Sauce thickener (4)
I believe the answer is:
roux
'sauce thickener' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for roux that I've seen before include "Fat, flour sauce base" , "Thickener for sauces" , "Mixture of fat and flour used as a base for sauces" , "Butter/flour mix" , "Butter and flour sauce combination" .)