Jumper for summer sport (7)
I believe the answer is:
cricket
I believe this clue is a double definition.
'jumper' is the first definition.
(crickets jump)
'summer sport' is the second definition.
(sport played in summer)
'for' acts as a link.
(Other definitions for cricket that I've seen before include "Game - "basically baseball on Valium" (Robin Williams)" , "Summer sport" , "little creature" , "short leg seen on this field?" , "Six-footer" .)