Make economies in vain (7)

I believe the answer is:

useless

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'vain' is the definition.
(I've seen this in another clue)

I cannot understand how the rest of the clue works.

(Other definitions for useless that I've seen before include "Unserviceable" , "idle" , "Serving no practical purpose" , "Good-for-nothing" , "Good for nothing" .)

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

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