Countermand the six-ball regulation (8)
I believe the answer is:
overrule
'countermand' is the definition.
(I've seen this before)
'the six-ball regulation' is the wordplay.
'the six ball' becomes 'over' (I can't explain this - if you can you should believe this answer much more).
'regulation' becomes 'rule' (synonyms).
'over'+'rule'='OVERRULE'
Can you help me to learn more?
(Other definitions for overrule that I've seen before include "Disallow a decision" , "Turn down (objection)" , "Set aside decision by superior authority" , "Disallow, set aside" , "Disallow by using one's superior authority" .)