Big portion of the Mongol Empire (4)
I believe the answer is:
gobi
'big portion of the mongol empire' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for gobi that I've seen before include "The . . . . Desert is a barren plateau of S Mongolia and N China" , "Plain" , "arid area" , "Desert in central Asia" , "Asian desert, Chinese name Shamo" .)