Some more volunteers picked up six balls (4)
I believe the answer is:
over
'six balls' is the definition.
(I've seen this before)
'some more volunteers picked up' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'picked up' shows that the letters should be reversed in order.
'revo' is hidden within 'more volunteers'.
'revo' in reverse letter order is 'OVER'.
(Other definitions for over that I've seen before include "Finished-up -left" , "Rouond of six bowls in cricket" , "Six consecutive bowls in cricket" , "Unit of test" , "Finished, done" .)