One marsh turning into desert (4)
I believe the answer is:
gobi
'desert' is the definition.
(I know that Gobi is a desert)
'one marsh turning' is the wordplay.
'one' becomes 'i' (Roman numeral).
'marsh' becomes 'bog' (a bog is a marshy area).
'turning' says the letters should be written backwards.
'i'+'bog'='ibog'
'ibog' backwards is 'GOBI'.
'into' acts as a link.
(Other definitions for gobi that I've seen before include "Desert in China/Mongolia; cauliflower (Ind.)" , "Mongolian/Chinese desert" , "The . . . . Desert is a barren plateau of S Mongolia and N China" , "very dry place" , "Asian desert region" .)