Victory! Their seconds forfeited season (6)
I believe the answer is:
winter
'season' is the definition.
(winter is a season)
'victory their seconds forfeited' is the wordplay.
I cannot quite see how this works, but
'victory' could be 'win' (a win is a victory) and 'win' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for winter that I've seen before include "Cold period" , "period of discontent?" , "period of freeze?" , "Time for mittens and scarves?" , "Cold season" .)