Redeye wrote:
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....

The piece of wood will eventually fall right through after heavy foot
traffic going over it.

My approach is the same, but with the floorboard being chamfered it cant
fall through.

--
Sir Benjamin Middlethwaite