Summertime activity which requires seeding? (4,6)
I believe the answer is:
lawn tennis
This clue is a double definition.
'summertime activity' is the first definition.
(lawn tennis is popular in the summer)
'which requires seeding?' is the second definition.
(tennis players might be seeded)
(Other definitions for lawn tennis that I've seen before include "sport" , "Court game" , "court action results" , "Real alternative" , "Wimbledon game" .)