Question: The Curse:

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> **Clockwork Curse**  
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>  All Attack Rolls the cursed target makes replace their d20 roll with a 10 instead of rolling, as per the effect of Clockwork Amulet.
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Extra Details:

* This curse is expected to apply for at least a full in-game day.
* Encounters for the day will come from the Forest or Coastal Encounters (5-10) tables in Xanathar's Guide to Everything.
* The character will be part of a 5-player group.
* The character's attacks are their ideal method of dealing damage.

The Goal:

* This curse is meant to be a setback or gameplay-shift to the player, not a crippling of the player.
	+ I want the player to spend time affected by the curse to work with it or work around it.
* Since the players can decide not to remove it, the beneficial aspect of it should not be too strong.

The Question:

Is this curse too detrimental that it imposes more of a penalty than I expect? Is this curse too beneficial in the long-run? Or is it reasonably fair?

Answer: 
Seems like a serious debuff
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In terms of what the character can actually do -- you roll a 15 or over like 25% of the time. That makes for a hard fight, but you can burn resources and win it. This curse makes easy fights trivial, and hard fights impossible. You might be able to, as the DM, design around this -- make things harder by giving more low-level enemies, and totally avoiding high level enemies. But that sounds really tedious. 

Also, how will you deal with advantage? I suspect the expected value of a roll with advantage is about 15. If you give them a 15 on an advantaged attack, it might be more manageable (also more fun).
Situational. 
Sounds like it would make hitting most low to medium enemies easy, but make it impossible to hit a boss or demi-boss. 

So useful for a front liner who will be wading through canon fodder mobs. 
Not directly useful against a armored one or boss, but silver lining. Against those, it would encourage the player to use the "help" action to help their team hit the boss.