Part of Okefenokee Swamp (3)
I believe the answer is:
fen
'swamp' is the definition.
(thesaurus)
'part of okefenokee' is the wordplay.
'part of' says the answer is hidden in the clue.
'FEN' can be found hidden inside 'okefenokee'.
(Other definitions for fen that I've seen before include "Low marsh" , "Water-logged area of vegetation" , "English marsh" , "Low marshy area" , "Low marshy land" .)