That's a mighty big hole for filler - it's possible but it'll take ages
to dry and quite possibly crack once it is dry.

I'd fashion another piece of wood of roughly the same size and shape as
the gap you want to fill, apply liberal amounts of PVA, then put the
wood in the gap and apply some kind of pressure for at least 12 hours,
then sand down as necessary to get the surface flat again.

To be honest, replacing the floorboard is probably the quickest and
easiest solution....