Yes, it's fine to answer just part of the question, especially if it's complex. From How to Answer:

Any answer that gets the asker going in the right direction is helpful, but do try to mention any limitations, assumptions or simplifications in your answer. Brevity is acceptable, but fuller explanations are better.

If you've got a solution to part of the problem, that 'nudge' in the right direction might be all they need. There's no use keeping that knowledge to yourself, so posting an answer will help everyone involved.

Using an answer is the right thing here—comments are for clarification / critique of a post, which your post would not be, and the only other option is to not post the answer at all. If you can help the author get closer to the answer, do so. Back up what you can with references, and be sure to mention what you couldn't find out the answer to, so others coming to the post can answer if they know more.

Finding the answer to hard questions might be a collaborative effort between multiple users—this is a great thing, so don't be afraid to get the ball rolling!

Yes, it's fine to answer just part of the question, especially if it's complex. From How to Answer:

Any answer that gets the asker going in the right direction is helpful, but do try to mention any limitations, assumptions or simplifications in your answer. Brevity is acceptable, but fuller explanations are better.

If you've got a solution to part of the problem, that 'nudge' in the right direction might be all they need. There's no use keeping that knowledge to yourself, so posting an answer will help everyone involved.

Using an answer is the right thing here—comments are for clarification / critique of a post, which your post would not be, and the only other option is to not post the answer at all. If you can help the author get closer to the answer, do so. Back up what you can with references, and be sure to mention what you couldn't find out the answer to, so others coming to the post can answer if they know more.

Finding the answer to hard questions might be a collaborative effort between multiple users—this is a great thing, so don't be afraid to get the ball rolling!