Treats badly in a state of inertia (6)

I believe the answer is:

atrest

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

'in a state of inertia' is the definition.
I can't tell whether this defines the answer.

'treats badly' is the wordplay.
'badly' indicates anagramming the letters (letters in the wrong order).
'treats' anagrammed gives 'ATREST'.

Can you help me to learn more?

I've seen this clue in The Irish Independent.

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