Clarity is a non-targeted Summoner Spell. It is only available on Howling Abyss. This spell instantly restores a portion of mana for yourself and all nearby allied champions.
Clarity is best taken in teams where most, if not all, champions are mana based. Usually this spell is taken by a mana-hungry champion that needs to compensate for low base mana/mana regen (e.g. Malphite), or focuses on aggressive play by spamming their skills (e.g. Ryze).