Loathsome ignoramus makes off, initially in car (4)
I believe the answer is:
limo
'car' is the definition.
(limo is a kind of car)
'loathsome ignoramus makes off initially' is the wordplay.
'initially' indicates taking the first letters.
The first letters of 'loathsome ignoramus makes off' is 'LIMO'.
'in' is the link.
(Other definitions for limo that I've seen before include "Luxury car" , "Large luxurious car" , "Stretched car, often used for partying" , "Stretched car (abbr)" , "Set of rooms" .)