Big cat -- and what it might do, wanting prey at first? (5)

I believe the answer is:

ounce

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'big cat' is the definition.
(I know that ounce is a type of big cat)

I don't understand the rest of the clue.

(Other definitions for ounce that I've seen before include "one of several in a pound" , "16 for a pound" , "Small amount (of old pound)" , "Jaguar" , "Large cat" .)

I've seen this clue in The Telegraph.

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