Old person at one point on a highway (6)
I believe the answer is:
dotard
'old person' is the definition.
(dotard is a kind of old person)
'one point on a highway' is the wordplay.
'one point' becomes 'dot' (synonyms).
'on' means one lot of letters go next to another.
'highway' becomes 'rd'.
'dot'+'a'+'rd'='DOTARD'
'at' acts as a link.
(Other definitions for dotard that I've seen before include "Weak old person" , "Person old and feeble" , "Person whose age has impaired their intellect" , "Senior citizen" , "Someone showing the weakness of old age" .)