Defensive blunder by father leaves one hopping! (4)
I believe the answer is:
frog
'one hopping' is the definition.
(frogs jump)
'defensive blunder by father' is the wordplay.
'defensive blunder' becomes 'og' (own goal, football blunder).
'by' means one lot of letters go next to another.
'father' becomes 'fr' (abbreviation as in a priest's title).
'og' put after 'fr' is 'FROG'.
'leaves' is the link.
(Other definitions for frog that I've seen before include "One of Ranidae" , "Croaking amphibian" , "Batrachian" , "Grown-up tadpole" , "fastener" .)