They're of pretty resilient material (5)

I believe the answer is:

tyres

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'they're' is the definition.
The definition suggests an adverb but the answer is not.

'of pretty resilient material' is the wordplay.
'of' says the answer is hidden in the clue.
'TYRES' can be found hidden inside 'pretty resilient material'.

Can you help me to learn more?

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

I've seen this clue in the Evening Standard.

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