An ironclad excuse for not buying luxuries (8)

I believe the answer is:

hardship

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'an ironclad excuse' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.

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

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(Other definitions for hardship that I've seen before include "Austerity - affliction" , "Severe discomfort, a tough time" , "Adversity, grimness" , "Privation difficult to endure" , "difficult to take" .)

I've seen this clue in The Globe and Mail.

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