Man behind slow vehicle (6)
I believe the answer is:
hearse
'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).
'he'+'arse'='HEARSE'
(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" .)