Firm got inventory prepared by Spooner (4,6)

I believe the answer is:

rock steady

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'firm' is the definition.
(I've seen this before)

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

(Other definitions for rock steady that I've seen before include "Without a tremor" , "firm" , "slow dance" , "Consistent" .)

I've seen this clue in The Hindu.

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