A clash sequence can be done once per match after you've lost your first bar of health and only have red health left. Start a clash by pressing Forward and the Meter Burn button at the same while you're stuck in a combo. (+ by default)
This starts a mini-game where you wager your super meter. Win the clash by betting more super meter than your opponent. If you both bet the same amount, it'll end in a tie. Bet the maximum amount you have to increase your chances of winning.
The AI will activate clashes often, so it's very likely you'll unlock this through normal gameplay during story or battle mode. Just remember to save your meter to make sure you have enough to win.
If for whatever reason you can't unlock it normally, there is an easy method using Practice Mode:
Enter practice mode, choose any characters and any stage.
Hit START, select PRACTICE OPTIONS, set CLASH to ON, and METER to FULL.
Next, select AI OPTIONS, set the enemy type to RECORD and hit .
Do chain combo and hit on your controller and it'll playback your recording. (I used the , , combo with Joker. Make sure all 3 hits combo.)
Now walk up to the recording and let it land the combo on you. Press + to interrupt the combo with a clash. Bet all your meter. You'll win and unlock the achievement.