Terrible man starting inquiry over commercial vehicle (4)
I believe the answer is:
ivan
'terrible man' is the definition.
(Ivan the Terrible - Ivan IV of Russia)
'starting inquiry over commercial vehicle' is the wordplay.
'starting' suggests taking the first letters.
'over' says to put letters next to each other.
'commercial vehicle' becomes 'van' (I've seen this before).
The initial letter of 'inquiry' is 'i'.
'i'+'van'='IVAN'
(Other definitions for ivan that I've seen before include "his progress in history was terrible!" , "Tsar - 197> the Terrible" , "- - the Terrible (Czar)" , "Name of six Russian tzars" , "Vain (anag) - Tsar of Russia" .)