Six balls might knock them for six (9)
I believe the answer is:
overpower
'knock them for six' is the definition.
I can't tell whether this definition defines the answer.
'six balls might' is the wordplay.
'six balls' becomes 'over' (cricket).
'might' becomes 'power' (being mighty).
'over'+'power'='OVERPOWER'
Can you help me to learn more?
(Other definitions for overpower that I've seen before include "Crush" , "Conquer" , "Subjugate" , "Get the better of" , "Subdue, vanquish" .)