Wetland plant has no small advantage (4)
I believe the answer is:
edge
'advantage' is the definition.
(I've seen this before)
'wetland plant has no small' is the wordplay.
'wetland plant' becomes 'sedge' (I've seen this before).
'has no' suggests deleting specific letters.
'small' becomes 's' (abbreviation - e.g. clothes size).
'sedge' with 's' removed is 'EDGE'.
(Other definitions for edge that I've seen before include "Demarcation" , "Keenness" , "Rim - move gradually" , "Side, brink" , "The part farthest from the middle" .)