A cleric's spell slot can hold a spell of its level or of a lower level.
A cleric can prepare a lower-level spell in a higher-level slot, just as any other spellcaster can. If the cleric's Wisdom score won't allow her to prepare spells in her higher-level slots, she still can use those slots for lower-level slots. For example, a 9th-level cleric has 4th-level spell slots available. If the cleric has a Wisdom score of only 13, however, she can prepare and cast up to 3rd-level spells only. She can prepare spells of 3rd level or lower in her otherwise unusable 4th-level spell slots. Although the text on page 32 in the Player's Handbook seems to imply that a domain spell slot can hold a spell of its own level only, there's no good reason to bar a cleric from preparing a lower-level spell from one of her domains for that slot.
Like other spellcasters, clerics also can prepare spells using metamagic feats, which make those spells use higher-level spell slots. To prepare or cast a spell modified with most metamagic feats, the cleric's Wisdom score must be at least equal to 10 + the spell's unmodified level. For example, a cleric with a Wisdom score of 14 can use a 5th-level spell slot to prepare a 4th-level spell modified with the Enlarge Spell metamagic feat (which makes the spell use a slot one level higher than normal). The Heighten Spell metamagic feat actually raises the spell's level, as noted in the D&D FAQ. To prepare or cast a spell modified with the Heighten Spell feat, the cleric's Wisdom score must be equal to 10 + the spell's modified level
and, just in case you want to read the whole article:
All about Clerics, part 2
yea, i know, i was wrong....but at least i'm admitting to it!