One marsh, when turned over, becomes a desert (4)
I believe the answer is:
gobi
'a desert' is the definition.
(I know that Gobi is a desert)
'one marsh when turned over' is the wordplay.
'one' becomes 'i' (Roman numeral).
'marsh' becomes 'bog' (a bog is a marshy area).
'when turned' shows that the letters should be reversed in order.
'over' is a charade indicator (letters next to each other) (in a down clue, some letters go over others).
'bog' written backwards gives 'gob'.
'i' after 'gob' is 'GOBI'.
'becomes' is the link.
(Other definitions for gobi that I've seen before include "Mongolian expanse" , "East Asian desert" , "Sandy area" , "Large desert plateau of southern Mongolia and northern China" , "Area of sand" .)