Stop at the base (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)
'the base' is the second definition.
(thesaurus)
'at' acts as a link.
(Other definitions for stem that I've seen before include "Slender vertical part, glass or plant" , "Tube" , "Halt flow" , "race" , "Thwart; support" .)