One commercial vehicle is Russian (4)
I believe the answer is:
ivan
'is russian' is the definition.
(I've seen this before)
'one commercial vehicle' is the wordplay.
'one' becomes 'i' (Roman numeral).
'commercial vehicle' becomes 'van' (I've seen this before).
'i'+'van'='IVAN'
(Other definitions for ivan that I've seen before include "'Terrible' emperor of Russia" , "First tsar of Russia" , "First Russian tsar, 1547-84" , "Vain (anag) - Tsar of Russia" , "in Russian history" .)