Spares among the off-duty reserves? (5)

I believe the answer is:

tyres

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'spares' is the definition.
The definition and answer can be both man-made objects as well as being plural nouns.
Perhaps there's an association between them I don't understand?

'among the off-duty reserves?' is the wordplay.
'among' says the answer is hidden in the clue.
'TYRES' can be found hidden inside 'off duty reserves'.

Can you help me to learn more?

(Other definitions for tyres that I've seen before include "Wheel hoops" , "Rubber wheel covers" , "Wheel coverings" , "rubber rings" .)

I've seen this clue in the Evening Standard.

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