Roams foolishly over southern marsh (6)
I believe the answer is:
morass
'marsh' is the definition.
(thesaurus)
'roams foolishly over southern' is the wordplay.
'foolishly' indicates anagramming the letters.
'over' indicates putting letters inside.
'southern' becomes 's' (abbreviation).
'roams' anagrammed gives 'moras'.
'moras' enclosing 's' is 'MORASS'.
(Other definitions for morass that I've seen before include "Slough - marsh" , "Bog or marsh" , "Tract of soft, wet ground" , "Bog or swamp" , "Bog; confused situation" .)