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 being comboed. (forward+RT 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.
It's likely you'll unlock this through normal gameplay, however there is an easy method using Practice Mode.
-Enter practice mode, choose any characters and any stage.
-Hit START, select PRACTICE OPTIONS, and set CLASH to ON.
-Next, select AI OPTIONS, set the enemy type to RECORD and hit A.
-Do chain combo and hit the BACK button on your controller and it'll playback your recording. (I used the X, X, Y combo with Joker. Make sure all 3 hits combo.)
-Now walk up to the recording and let it land the combo on you. Press forward+RT to interrupt the combo with a clash. Bet all your meter. You'll win and unlock the achievement.