A substantial proportion may get hit on the road (5)
I believe the answer is:
third
'a substantial proportion' is the definition.
The definition and answer can be both relationships as well as being singular nouns.
Perhaps you can see an association between them that I don't see?
'hit on the road' is the wordplay.
I cannot really understand how this works, but
an anagram of 'hit' is 'thi' which is located in the answer.
'road' could be 'rd' (abbreviation) and 'rd' is found in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
'may get' is the link.
Can you help me to learn more?
(Other definitions for third that I've seen before include "degree" , "Ordinal number" , "Bronze medal position" , "Less than half, rather" , "Eroica, Beethoven's - symphony" .)