The spell must be on your spell list; otherwise, you can't even read what the spell scroll says
The description of the spell scroll magic item (DMG, p. 200) begins:
A spell scroll bears the words of a single spell, written in a mystical cipher. If the spell is on your class’s spell list, you can read the scroll and cast its spell without providing any material components. Otherwise, the scroll is unintelligible. [...]
In this case, the power word stun spell is on the bard, sorcerer, warlock, and wizard spell lists. However, since you're playing a cleric, your character would not be able to read this spell scroll in the first place, let alone use it.
But let's say that at least one person in your party is a bard, sorcerer, warlock, or wizard. They then try to use the spell scroll, but the spell's of a higher level than they can cast (I'm guessing they're 10th-level characters as well). What are the rules for this?
If it's on your spell list, you need to pass a spellcasting ability check (DC = 10 + spell level) to use the spell scroll
As noted in Purple Monkey's answer, the second paragraph of the spell scroll magic item description adds:
If the spell is on your class's spell list but of a higher level than you can normally cast, you must make an ability check using your spellcasting ability to determine whether you cast it successfully. The DC equals 10 + spell's level. On a failed check, the spell disappears from the scroll with no other effect.
Assuming you have an ally who has the spell on their spell list, they can attempt to use the spell scroll. To successfully cast the spell from the scroll, they must make an ability check using their spellcasting ability; the DC for this check is equal to 10 + the level of the spell on the scroll.
In the case of a spell scroll of power word stun, the DC for the spellcasting ability check would be 18. (A bard, sorcerer, or warlock would make a DC 18 Charisma check – since Charisma is their spellcasting ability – while wizards would have to succeed on a DC 18 Intelligence check.)
Note that regardless of whether the character succeeds or fails on this spellcasting ability check, the spell scroll is consumed/destroyed after it's used.