One swamp turned to desert (4)
I believe the answer is:
gobi
'desert' is the definition.
(I know that Gobi is a desert)
'one swamp turned' is the wordplay.
'one' becomes 'i' (Roman numeral).
'swamp' becomes 'bog' (both can mean a wet or marshy area).
'turned' says the letters should be written backwards.
'i'+'bog'='ibog'
'ibog' in reverse letter order is 'GOBI'.
'to' acts as a link.
(Other definitions for gobi that I've seen before include "very dry place" , "Asian desert, Chinese name Shamo" , "Sandy area" , "Dry spot" , "Asian desert region" .)