Runner in race (5)
I believe the answer is:
shoot
This clue is a double definition.
'runner' is the first definition.
(runner is a type of shoot from a plant)
'race' is the second definition.
(shoot can mean to run or race off)
'in' is the link.
(Other definitions for shoot that I've seen before include "big game expedition?" , "Use a gun; film" , "Fire (a gun)" , "Fire at the new branch?" , "Make a film" .)