It could be that the ply is de-laminating and your crack is just the surface layer of ply. If this is the case you can strip the varnish from the effected area, let thoroughly dry and then stipple or squeegee some epoxy resin into the crack. Place a sheet of cling film over the glued area and clamp a flat piece of timber over the whole lot. When dry you should have sealed the crack.
Matt
W773