See posh car followed by unknown vehicle (5)
I believe the answer is:
lorry
'vehicle' is the definition.
(lorry is a kind of vehicle)
'see posh car followed by unknown' is the wordplay.
'see' becomes 'lo' ('lo' is an archaic exclamation meaning 'look!').
'posh car' becomes 'rr' (short for Rolls Royce).
'followed by' says to put letters next to each other.
'unknown' becomes 'y' (algebra).
'lo'+'rr'+'y'='LORRY'
(Other definitions for lorry that I've seen before include "HGV" , "From which stolen goods fall?" , "Commercial vehicle" , "Heavy goods vehicle" , "Large motor vehicle for transporting goods" .)