It depends on what you mean by "cleric".
If you mean a person in the party who functionally provides healing, support, and information of a kind other spellcasters can't, then as long as you have a mage subtype who can do this, you'll be fine without having to name a specific class for it.
If you mean a person who is tuned into the metaphysical aspects of the campaign world, i.e. knows about the setting's Powers and can provide meaningful instructions from/information about them, then not all settings will need those in the style we know them, but I admit I am one of the people who finds human-based cultures that don't have a religion of some sort to fail my plausibility test -- they may not need to be more than NPCs for a particular game, but they should at least be around.