Source of flour used to make dough? (4,4)

I believe the answer is:

cash crop

'source of flour' is the definition.

I don't understand how the rest of the clue works.

(Other definitions for cash crop that I've seen before include "Profitable farm produce" , "Harvest for sale rather than consumption" , "Commercial farm product" , "What the grower will sell" , "Ready-saleable harvest" .)

I've seen this clue in The Times.

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