Missing cord that's useful for a jumper? (8-4)
I believe the answer is:
skipping-rope
'that's useful for a jumper?' is the definition.
(someone who jumps may use a skipping rope)
'missing cord' is the wordplay.
'missing' becomes 'skipping' (skipping is a kind of missing).
'cord' becomes 'rope' (rope is a kind of cord).
'skipping'+'rope'='SKIPPING-ROPE'
(Other definitions for skipping-rope that I've seen before include "It is swung over head and under foot" , "Cord (used by playing children)" , "Cord that's twirled and jumped" , "Cord with handles for jumping over" .)