Start betting cartel (6)
I believe the answer is:
spring
'start' is the definition.
(both can mean to jump or be startled)
'betting cartel' is the wordplay.
'betting' becomes 'sp'.
'cartel' becomes 'ring' (both can mean a criminal group).
'sp'+'ring'='SPRING'
(Other definitions for spring that I've seen before include "Schumann symphony" , "Leap - part of year" , "Bound < to get out of prison" , "early in the year" , "Leap, well" .)