Witness Serb dancing in six balls (8)
I believe the answer is:
observer
'witness' is the definition.
(I've seen this before)
'serb dancing in six balls' is the wordplay.
'dancing' indicates anagramming the letters.
'in' means one lot of letters goes inside another.
'six balls' becomes 'over' (sequence of six balls bowled in cricket).
'serb' with letters rearranged gives 'bser'.
'bser' placed within 'over' is 'OBSERVER'.
(Other definitions for observer that I've seen before include "Looker-on" , "broadsheet" , "Sunday newspaper" , "One watching, evaluating" , "He sees" .)