Irate mum snaps. "Childish!" (8)

I believe the answer is:

immature

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'childish' is the definition.
(similar in meaning)

'irate mum snaps' is the wordplay.
'irate mum' can be anagrammed to 'IMMATURE'.
However, I am unsure how this is indicated.

Can you help me to learn more?

(Other definitions for immature that I've seen before include "Not fully ripe or developed" , "Not yet fully-developed" , "Not yet adult or fully grown" , "Not yet fully grown or developed" , "Young" .)

I've seen this clue in the Sydney Morning Herald.

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