You really need to grain fill Ash anyway, as it's an open pore wood. But it seems to me that there's a sizeable split in the wood, and simple filling isn't going to be the answer as it needs glueing down. The trouble with glueing is that if you get any on the surface, then the glue needs to be removed otherwise the stain won't take in that area.

I haven't done anything similar , so am not the best person to give guidance here (I've seen YouTube videos of similar things, but there's nothing like having done it yourself), but I'd suggest using some PVA (or red-label Titebond) glue to glue up the crack. You need a glue that's water soluble.

First wet the crack area and get water down into the crack. Open the crack up with a thin blade if you can. The water will help thin the PVA and get it into the crack.

Then carefully push PVA glue into the crack - a small artists paintbrush is recommended. Wet the area again and repeat. Push the crack closed, to squeeze out excess glue. You then need to use a wet lint-free rag and dab/wipe as much of the glue off the surface as you can (making sure not to simply spread the glue out over a larger area). Then I'd cover the area with some cling-film or a bit of a plastic bag that the glue won't stick to well, then using a small wood block either side of the body, clamp the crack tight.

Leave overnight, then unclamp and I'd then let dry for a bit longer. Hopefully the crack should have gone. The water will have raised some grain, so I'd sand the general area again, maybe try and remove the local dishing. This should help remove any glue on the surface but there still may be some resudue, so I'd so a wetting check with metho or turps. If this shows any glue spots, you can either try a bit more sanding or else use some Goof-Off.

I'd use a neutral grain filler (I'd suggest Timbermate) just on the crack area first (you'll only need a very small amount), then let that dry and sand back flat. I'd then use a darker grain filler over all the body to give it a level surface and highlight the grain pattern. Timbermate is water-based, so you can use a water-based dye to colour the a neutral grain filler, or you can simply buy a tin of neutral and stained Timbermate if you don't want to mess about.

I'm suggesting using the neutral timbermate on the crack area as it will help prevent the darker Timbermate from highlighting the crack, but I'd still try and avoid putting darker timbermate on the crack itself.

When applying the Timbermate, you really need to push it into the wood pores whilst scraping it off the surface to leave as thin a later as possible to be sanded back to wood (you only want the grain filler to fill the pores, not cover the surface). An old credit card is often mentioned for this task, but you can also buy rubber-edged spreaders, which are probably slightly better, having a bit more 'give'.

I'm not saying that the above is the best solution, but I think it's a reasonable one. If there are any flaws, then hopefully others will point them out.