Crossword Genius

Little energy in high Inca capital, lacking oxygen flow rate? (5)

Ross

I believe the answer is:

cusec

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'rate?' is the definition.
Both the definition and answer are singular nouns.
Maybe there's a link between them I don't understand?

I cannot understand how the rest of the clue works.

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(Another definition for cusec that I've seen is " Measure of flux".)

I've seen this clue in The Times and The Sunday Times.
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