One marsh turns up in desert (4)
I believe the answer is:
gobi
'desert' is the definition.
(I know that Gobi is a desert)
'one marsh turns up' is the wordplay.
'one' becomes 'i' (Roman numeral).
'marsh' becomes 'bog' (a bog is a marshy area).
'turns up' shows that the letters should be reversed in order.
'i'+'bog'='ibog'
'ibog' back-to-front is 'GOBI'.
'in' acts as a link.
(Other definitions for gobi that I've seen before include "barren tract" , "Asian desert region" , "Large, mainly Mongolian, desert" , "Desert in central Asia" , "Large desert plateau of southern Mongolia and northern China" .)