One marsh to north in vast arid expanse (4)
I believe the answer is:
Here is my best explanation:
'vast arid expanse' is the definition.
(I've seen this before)
'one marsh to north' is the subsidiary indication.
'one' becomes 'i' (Roman numeral).
'marsh' becomes 'bog' (I've seen this before).
'to north' says the letters should be written in reverse.
'ibog' reversed gives 'gobi'.
'in' is the link.
(Other definitions for gobi that I've seen before include "Desert of central China", "East Asian desert", "vast arid expanse", "Desert in China/Mongolia; cauliflower (Ind.)", "China/Mongolia desert", "Desert of Mongolia and northern China", "barren tract", "Asian desert region", "Asiatic barren plateau which means 'stony desert'" and "Area of sand".)