Regret game ending in melee (3)
I believe the answer is:
rue
'regret' is the definition.
(I've seen this before)
'game ending in melee' is the wordplay.
'game' becomes 'ru' (rugby union).
'ending in' indicates one should take the final letters.
The last letter of 'melee' is 'e'.
'ru'+'e'='RUE'
(Other definitions for rue that I've seen before include "Bemoan" , "Swann's way?" , "Bitterly regret - plant" , "Feel bad" , "Cry" .)