Staunch supporter of plant (4)
I believe the answer is:
I believe this is a double definition.
'staunch' is the first definition.
(I know that staunch can be written as stem)
'supporter of plant' is the second definition.
(a stem is a part of a plant)
(Other definitions for stem that I've seen before include "supporter of tobacco?" , "Staunch (flow)" , "Stanch" , "Upward part of a plant" , "something growing?" .)