Big cheese Diana Ross once cut to get slice of Ossau-Iraty (7)

I believe the answer is:

supremo

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'big cheese' is the definition.
(I've seen this before)

I cannot understand how the rest of the clue works.

(Other definitions for supremo that I've seen before include "C-in-C" , "Big fish" , "Person in overall charge, big boss" , "Champion" , "Person invested with great authority" .)

I've seen this clue in The Independent.

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