Singular blade for cutting marsh plant (5)
I believe the answer is:
sedge
'marsh plant' is the definition.
(sedge is a kind of marsh plant)
'singular blade for cutting' is the wordplay.
'singular' becomes 's' (abbreviation sometimes used in dictionaries).
'blade for cutting' becomes 'edge'.
's'+'edge'='SEDGE'
(Other definitions for sedge that I've seen before include "plant with triangular stems" , "Bog plant; type of warbler" , "Herons" , "Plant like grass" , "fly by rivers" .)