A form of life in the upper atmosphere (3)

I believe the answer is:

rat

I'm a little stuck... Click here to teach me more about this clue!

'a form of life' is the definition.
I can't judge whether this definition defines the answer.

'in the upper atmosphere' is the wordplay.
'in' indicates a hidden word.
'RAT' is hidden within 'upper atmosphere'.

Can you help me to learn more?

(Other definitions for rat that I've seen before include "Inform" , "Abandon" , "one escaping doomed vessel?" , "Change sides" , "Contemptible person" .)

I've seen this clue in the Evening Standard.

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