Restrain a supporter (4)
I believe the answer is:
stem
This clue is a double definition.
'restrain' is the first definition.
(I've seen this in another clue)
'a supporter' is the second definition.
(I've seen this before)
(Other definitions for stem that I've seen before include "Tube" , "Main central part as of plant or glass" , "Central upright part of flower or drinking glass" , "section of pipe" , "Ski turn and plant stalk" .)