This was here when I got it. I noticed it. To me, it looks like the glue joint wasn't perfectly flat when they glued the body slabs together. I can see dry glue in the joint, and it's not that deep. Maybe .03" or so. I was waiting for the wood to react to my environment and see if it gets worse. So far, so good. What should I fill it with?
If you have the right clamps, then you could see if you can close the gap. If you can, then wood glue and clamp it up.
If the gap won’t close I suppose it depends on the finish. If you plan to stain, then you don't want t get anything that's going to soak into the body and reject the stain. Though you could stain first and then wick in some thin CA,
If it all seems stable and isn't enlarging, you could just use a grain/wood filler coloured to match the wood to fill the crack. or you could wick some CA in and then fill the rest of the crack.
If you were painting it a solid colour, then maybe fill it with epoxy.
Think I'm going with the stain, then CA. It looks to me like the plank was moved, or unsupported before fully pulled out of the planer. The body joint is perfectly straight until the last inch. The gap is about .01" wide at its widest point.
+1 on the CA. An alternative after staining is to mask either side of the crack and I found that epoxy can also seep into cracks , especially if you can use a card or something to swipe across the crack. I was building a canoe at the time I did this but CA would be more accesable.