Slough's roundabout, one in desert (4)
I believe the answer is:
gobi
'desert' is the definition.
(I know that Gobi is a desert)
'slough's roundabout one' is the wordplay.
'slough' becomes 'bog' (both can mean a swampy area).
'roundabout' says the letters should be written in reverse (letters go around).
'one' becomes 'i' (Roman numeral).
'bog' back-to-front is 'gob'.
'gob'+'i'='GOBI'
'in' acts as a link.
(Other definitions for gobi that I've seen before include "Plain" , "Sandy area" , "Large Far Eastern desert" , "barren tract" , "Dry spot" .)