This answer might be contrary to official opinion/community consensus on SE, but I still think that there are a few options.
If you have some energy and willingness to work with what might even be a defensive, frustrated, or unmotivated student, then perhaps the following might be helpful.
In the comments section demand to know the source of the problem, statements of definitions of major terms, and perhaps what is making the problem difficult/challenging/interesting.
Provide in the comments a quick hint ("if $f(x) \in X$ and $X \subseteq Y$, then $f(x) \in Y$. Can you see why this helps?")
Direct OP to similar questions (these "no clue" types are usually duplicates) and ask that they respond as to whether or not the other answers were helpful.
In general, I'd say one of the three methods works roughly $40$ percent of the time. Usually at further solicitation, OP will mumble something (usually wrong) in the comments section, at which point you can ask them to put it in their original question. If that doesn't work, then you gave it your best and you can "let the question die."
If you choose to give a full solution, I think that it's worth mentioning that the solution is probably only useful for future readers. It adds to the collection of problems/solutions on the website. Depending on how highly you estimate the utility of this, it might be worth doing. I've done it if I find the problem interesting for whatever reason and wanted to share my solution. (I'm not a mathematician, but a student, so I don't think that's so evil.)