Everyone attending event after jumping barrier on racecourse (8,4)
I believe the answer is:
starting gate
'barrier on racecourse' is the definition.
'everyone attending event after jumping' is the wordplay.
'everyone attending event' becomes 'gate' (term for the crowd at a sporting event).
'after' says to put letters next to each other.
'jumping' becomes 'starting' ('start' can be a synonym of 'jump').
'gate' after 'starting' is 'STARTING-GATE'.
(Other definitions for starting gate that I've seen before include "opening for filly" , "Barrier lifted when race begins" , "barrier to progress before race begins" , "Moveable barrier on racecourses" .)