Hastily shift marsh plant (4)
I believe the answer is:
rush
This clue is a double definition.
'hastily shift' is the first definition.
(to rush is to move hastily)
'marsh plant' is the second definition.
(I know that rush is a type of marsh plant)
(Other definitions for rush that I've seen before include "High" , "Water plant; hurry" , "Traffic is busiest at this hour" , "plant developing on shore" , "Race" .)