Unknown person runs into road rage (8)
I believe the answer is:
stranger
'unknown person' is the definition.
(I've seen this before)
'runs into road rage' is the wordplay.
'runs' becomes 'r' (cricket abbreviation).
'into' means one lot of letters goes inside another.
'road' becomes 'st'.
'rage' becomes 'anger' (I've seen this before).
'st'+'anger'='stanger'
'r' going inside 'stanger' is 'STRANGER'.
(Other definitions for stranger that I've seen before include "Newcomer, alien" , "Non-acquaintance" , "More odd or wierd" , "Person in place or company to which he does not belong" , "One who is little known" .)