Cancel Rugby Union contest (3,3)
I believe the answer is:
rub out
'cancel' is the definition.
(I know that rub out is a type of cancel)
'rugby union contest' is the wordplay.
'rugby union' becomes 'RU'.
'contest' becomes 'bout' (I've seen this in another clue).
'ru'+'bout'='RUB OUT'
(Other definitions for rub out that I've seen before include "Kill (slang)" , "murder" , "Erase -- kill" , "Delete" , "'Erase (3,3)'" .)