Lightweight cat (5)

I believe the answer is:

ounce

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But I can't explain it at all! Could you help me understand?

(Other definitions for ounce that I've seen before include "Jaguar" , "One-sixteenth of pound" , "Twelfth of a pound" , "Fraction of a pound" , "one of a number in the pound" .)

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

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