A part in which to make a hit on the road (5)
I believe the answer is:
third
'a part' is the definition.
(fraction)
'to make a hit on the road' is the wordplay.
'to make' is an anagram indicator.
'on' means one lot of letters go next to another.
'the road' becomes 'rd' (abbreviation for road).
'hit' anagrammed gives 'thi'.
'thi'+'rd'='THIRD'
'in which' is the link.
(Other definitions for third that I've seen before include "Next after second" , "Bronze prize position" , "Less than half, rather" , "Ordinal number" , "Last of three" .)