So basically, I've found Clarity useful for being able to spam spells and do a great deal of damage as well as boosting other champions before or during teamfights when mana can become scarce. If a team needs little mana or has low usage on burst, then go with Ignite or another spell of your choice. Recently, I've been using Ignite more often for the finishing effect early game.

Concerning the choice of Frozen Mallet, I chose it for the slow which allows Twitch to keep enemies in range of his bow and keep them from escaping, and for the health boost, which I've found very useful in mid to late-game.

Clarity... Frozen Mallet... No.