Start getting judge to sit astride horse (7)
I believe the answer is:
trigger
'start' is the definition.
(I've seen this before)
'judge to sit astride horse' is the wordplay.
'judge' becomes 'trier' (a judge tries a case in court).
'to sit astride' indicates putting letters inside (some letters go across or astride others).
'horse' becomes 'gg' (geegee or GG is a childish term for a horse).
'trier' going around 'gg' is 'TRIGGER'.
'getting' is the link.
(Other definitions for trigger that I've seen before include "Start" , "Gun release mechanism" , "Lever to activate firing mechanism" , "trip" , "Releasing device" .)