Weapon from the latter part of the Iron Age (6)

I believe the answer is:

onager

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'weapon' is the definition.
(I have seen 'ancient weapon' mean 'onager' so perhaps 'weapon' could also mean 'onager')

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for onager that I've seen before include "Asian wild ass - orange (anag)" , "Asiatic wild ass" , "wild beast" , "The ass is a strange orange" , "Strangely, the wild ass might be orange" .)

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

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