On the road, their tread is endless (5)
I believe the answer is:
tyres
'road their tread is endless' is the definition.
I can't judge whether this definition defines the answer.
'on the' is the wordplay.
I cannot really understand how this works, but
'on' could be 're' ('on' can be mean 'with reference to') and 're' is located in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for tyres that I've seen before include "rubber rings" , "Wheel hoops" , "Wheel coverings" , "Rubber wheel covers" .)