Check, curb (4)
I believe the answer is:
stem
I believe this clue is a double definition.
'check' is the first definition.
(both can mean to control or stop)
'curb' is the second definition.
(I've seen this in another clue)
(Other definitions for stem that I've seen before include "race" , "supporter of flag, for example" , "Originate (from)" , "Stalk; restrict (a flow)" , "Main stalk of a plant" .)