So unnecessary to economise (7)

I believe the answer is:

useless

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'so' is the definition.
Both the definition and answer are adjectives. Perhaps they are linked in a way I don't understand?

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for useless that I've seen before include "Without purpose" , "Poor" , "Futile and unavailing" , "Lacking in purpose" , "Counterproductive" .)

I've seen this clue in the Sydney Morning Herald.

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