Jumper might go over this, missing tie (8,4)
I believe the answer is:
skipping rope
'jumper might go over this' is the definition.
(as in a person jumping)
'missing tie' is the wordplay.
'missing' becomes 'skipping' (skipping is a kind of missing).
'tie' becomes 'rope' (roping is a kind of tiing**).
'skipping'+'rope'='SKIPPING-ROPE'
(Other definitions for skipping rope that I've seen before include "toy" , "Cord with handles for jumping over" , "equipment used when jumping" , "Training aid" , "Item avoided during play" .)