Man behind slow vehicle (6)
'slow vehicle' is the definition.
(vehicle which moves slowly in a funeral procession)
'man behind' is the wordplay.
'man' becomes 'he'.
'behind' becomes 'arse' (synonyms).
(Other definitions for hearse that I've seen before include "Vehicle for carrying a coffin", "Car for carrying a coffin", "Undertaker's car", "Carriage for a coffin", "Funeral vehicle".)