Johnny-come-lately's winning jump (7)
I believe the answer is:
upstart
'johnny-come-lately's' is the definition.
(I've seen this in another clue)
'winning jump' is the wordplay.
'winning' becomes 'up' (eg being up at half time in football).
'jump' becomes 'start' (as in starting or jumping in fear).
'up'+'start'='UPSTART'
(Other definitions for upstart that I've seen before include "Brash newcomer" , "Pipsqueak" , "Person who behaves arrogantly in the new position" , "Person suddenly risen in world, and arrogant" , "mushroom" .)