Powerless to attack cat (5)

I believe the answer is:

ounce

'cat' is the definition.
(ounce can mean a snow leopard)

I cannot understand how the remainder of the clue works.

(Other definitions for ounce that I've seen before include "prowler" , "Big cat of Asia" , "Carnivore" , "Small imperial weight" , "A light imperial measure of weight" .)

I've seen this clue in The Times.

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