ORIGINAL: shouldn't be structural anyway.
If you must fill a gap in a structural joint, balsa dust (or baking soda) and CA seems to work OK, but I prefer epoxy as it is less brittle and easier to apply/work.
If you must fill a gap in a structural joint, DON'T. Rebuild the joint so you have solid wood-to-wood contact. Any joint that is not solid mechanically is not nearly as strong as one that is. The secret to good, strong building is good, solid joints.
Dr.1