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 tell whether this defines the answer.

'in the upper atmosphere' is the wordplay.
'in' says the answer is hidden in the clue.
'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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