Their tread, on the road, is wearing (5)
I believe the answer is:
tyres
'road is wearing' is the definition.
I can't judge whether this definition defines the answer.
'their tread on the' is the wordplay.
I cannot really see how this works, but
'on' could be 're' ('on' can be mean 'with reference to') and 're' is found within the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for tyres that I've seen before include "Wheel coverings" , "rubber rings" , "Wheel hoops" , "Rubber wheel covers" .)