Summer sport's jumper? (7)
I believe the answer is:
cricket
I believe this clue is a double definition.
'summer sport's' is the first definition.
(sport played in summer)
'jumper?' is the second definition.
(crickets jump)
(Other definitions for cricket that I've seen before include "Six-footer" , "English game or chirping insect" , "Summer sport" , "short leg seen on this field?" , "Insect - sport" .)