Border of the marsh (3)
I believe the answer is:
hem
'border' is the definition.
(I've seen this before)
'of the marsh' is the wordplay.
'of' says the answer is hidden in the clue.
'HEM' is hidden in the letters of 'the marsh'.
(Other definitions for hem that I've seen before include "Stitched edge of garment" , "Express hesitation" , "It draws attention to" , "Bottom of a garment" , "Part of skirt" .)