I would mask it off on either side of the offending gap & lay a bead of clear 100% silicone caulking in there. Then strike it down with a moist fingertip, pull the masking tape & carefully smooth it down, again with a moist finger tip.

But that's just me, I did carpentry for 20 yrs & that's how I roll...

If you do this, do it in a place where you are guaranteed a dust free environment for at least a few hours while the caulking sets up.

Another option would be clear acrylic latex caulking...it won't last as long, but is easier to remove if anything goes horribly wrong...

Nice lookin' pony...