Stop race (4)
I believe the answer is:
stem
I believe this is a double definition.
'stop' is the first definition.
(stemming is a kind of stopping)
'race' is the second definition.
(thesaurus)
(Other definitions for stem that I've seen before include "Main central part as of plant or glass" , "Upward part of a plant" , "Restrain the flow of" , "Dam" , "Central upright part of flower or drinking glass" .)