This is an idea stolen from Eric of Ptolus's Banewarrens game (though I am certain its origins lie much closer to the birth of D&D than that).
The camp cleric and fighter are a pair of NPC hirelings employed by the PCs to set-up camp, tend to the horses (or more exotic mounts), provide minor healing and spellcasting, stand guard, and run errands while the PCs explore the dungeon. They are of a level low enough to be outclassed if accompanying the party (so as to dissuade such accompaniment) but high enough to provide some benefit and withstand the expected rigors of the environment just outside the dungeon (or on a higher, and thus safer, level of the dungeon). In the abovementioned Banewarrens game, the party was level 6 while the brother and sister fighter and cleric duo were level 2. This means that a level appropriate encounter for the party (EL 6), one that would expend about 20% of the party's resources, would be overpoweringly difficult for an entire party of adventurers the same level as the hirelings (TPK).