Edge of the marsh (3)
I believe the answer is:
hem
'edge' is the definition.
(thesaurus)
'of the marsh' is the wordplay.
'of' indicates the answer is hidden within the clue.
'HEM' is hidden within 'the marsh'.
(Other definitions for hem that I've seen before include "may attract attention" , "Stitched edge of garment" , "A stitched border of cloth" , "Finished edge of a piece of cloth" , "try to attract attention" .)