One racing on the carriageway is cuckoo (10)
I believe the answer is:
roadrunner
'cuckoo' is the definition.
(I know that roadrunner is a type of cuckoo)
'one racing on the carriageway' is the wordplay.
I cannot really understand how this works, but
'one' could be 'un' (northern English dialect form of 'one') and 'un' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
'is' is the link.
Can you help me to learn more?
(Other definitions for roadrunner that I've seen before include "Chaparral cock" , "Bird" .)