Staunch supporter of plants (4)
I believe the answer is:
stem
This could be a double definition.
'staunch' is the first definition.
(I've seen this before)
'supporter of plants' is the second definition.
'stem' can be an answer for 'supporter' (I've seen this before). I am not sure about the 'of plants' bit.
Can you help me to learn more?
(Other definitions for stem that I've seen before include "feature of wine glass" , "Vertical part of flower or drinking glass" , "Axis" , "Stop; stalk; ship's timber" , "Main part of noun or verb" .)