They're of pretty resilient rubber (5)
I believe the answer is:
tyres
'they're' is the definition.
The definition suggests an adverb but the answer is not.
'of pretty resilient rubber' is the wordplay.
'of' indicates the answer is hidden within the clue.
'TYRES' can be found hidden inside 'pretty resilient rubber'.
Can you help me to learn more?
(Other definitions for tyres that I've seen before include "Wheel coverings" , "rubber rings" , "Rubber wheel covers" , "Wheel hoops" .)