Base for sauce? Alternative kept back by wife (4)

I believe the answer is:

roux

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'base for sauce?' is the definition.
(I've seen this before)

I can't explain the rest of the clue.

(Other definitions for roux that I've seen before include "sauce mix" , "Mixture of fat and flour used for thickening sauces" , "Cook's thickening" , "Fat/flour sauce-mix" , "Mixture of fat and flour used as a base for sauces" .)

I've seen this clue in The Times and The Sunday Times.

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