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.
Maybe you can see an association between them that I don't see?
'get hit on the road' is the wordplay.
'get hit on' becomes 'thi' (I can't justify this - if you can you should believe this answer much more).
'the road' becomes 'rd' (abbreviation).
'thi'+'rd'='THIRD'
'may' 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" .)