Blackguard’s jumper (7)
I believe the answer is:
bounder
This clue is a double definition.
'blackguard's' is the first definition.
(both can mean a dishonourable person)
'jumper' is the second definition.
(someone who bounds or leaps)
(Other definitions for bounder that I've seen before include "Untrustworthy type (once)" , "A hop-along cad?" , "despicable man" , "The skipper" , "Dishonourable man (dated colloq.)" .)