Macedonia is out of sodium carbonate and that's tough (6)

I believe the answer is:

robust

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'that's tough' is the definition.
(thesaurus)

I don't understand the rest of the clue.

(Other definitions for robust that I've seen before include "Having a strong constitution" , "No-nonsense" , "Vigorous and hearty" , "Tough!" , "of sturdy construction" .)

I've seen this clue in the Irish Times.

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