Dash of herb from the marsh (4)
I believe the answer is:
rush
'dash' is the definition.
(I know that dash is a more specific form of the action rush)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for rush that I've seen before include "Move fast or attack suddenly" , "Hurry, surge" , "Water plant; hurry" , "Hurry to the grassy plant" , "Drug-induced euphoria" .)