Regret rugby game's ending (3)
I believe the answer is:
rue
'regret' is the definition.
(to rue something is to regret it)
'rugby game's ending' is the wordplay.
'rugby' becomes 'RU'.
'ending' indicates one should take the final letters (the ending of the word).
The final letter of 'game' is 'e'.
'ru'+'e'='RUE'
(Other definitions for rue that I've seen before include "Feel regret for the herb" , "Be regretful" , "Feel remorse" , "French way" , "Plant producing oil" .)