One on spit in Mongolian plain (4)
I believe the answer is:
gobi
'plain' is the definition.
(I've seen this before)
'one on spit in mongolian' is the wordplay.
'one' becomes 'I' (Roman numeral).
'on' says to put letters next to each other.
'spit in mongolian' becomes 'gob' (I can't explain this - if you can you should give a lot more credence to this answer).
'i' put after 'gob' is 'GOBI'.
Can you help me to learn more?
(Other definitions for gobi that I've seen before include "Area of sand" , "Asian desert region" , "Desert of central China" , "vast arid expanse" , "Desert, mostly in Mongolia" .)