If you take a Prestige Class that gives "+1 spellcasting level in existing class", it applies to an existing class with its own list.
You can increase your spellcasting level as a Cleric, or a Druid, or a Ranger, or a Bard, or an Assassin, or a Sacred Fist; you can't increase your spellcasting level as a Loremaster, because they don't have their own level, they just modify one that already exists.
-Hyp.