In South America, a vehicle principally needed in treeless plain (7)
I believe the answer is:
savanna
'treeless plain' is the definition.
(I've seen this in another clue)
'in south america a vehicle principally needed' is the wordplay.
'in' is an insertion indicator.
'south america' becomes 'SA' (abbreviation).
'vehicle' becomes 'van' (type of vehicle).
'principally' indicates taking the first letters.
The initial letter of 'needed' is 'n'.
'a'+'van'+'n'='avann'
'sa' going around 'avann' is 'SAVANNA'.
'in' is the link.
(Other definitions for savanna that I've seen before include "Large grassy plain with few trees" , "Flat, tropical grassland" , "Grassy (African) plain" , "Open grasslands of Africa" , "African grassland" .)