Prepared papers on art, being extra useful (5,4)

I believe the answer is:

spare part

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'extra useful' is the definition.
I can't judge whether this defines the answer.

'prepared papers on art' is the wordplay.
'prepared' indicates an anagram.
'on' says to put letters next to each other.
'papers' is an anagram of 'sparep'.
'sparep'+'art'='SPARE-PART'

'being' is the link.

Can you help me to learn more?

(Other definitions for spare part that I've seen before include "Type of surgery" , "something put in machine?" , "Extra component for machine" , "Replacement component" .)

I've seen this clue in the Evening Standard.

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