Regret game ending in battle (3)
I believe the answer is:
rue
'regret' is the definition.
(to rue something is to regret it)
'game ending in battle' is the wordplay.
'game' becomes 'RU' (rugby union).
'ending in' says to take the final letters.
The last letter of 'battle' is 'e'.
'ru'+'e'='RUE'
(Other definitions for rue that I've seen before include "Lament; a herb" , "show regret" , "Regret the bitter herb" , "Regret in a French street" , "Bemoan" .)