Wood filler on plaster

No, don't use wood filler, use a multi purpose filler or easy fill.

Easi-fill would be best or just general polyfilla. You should ask yourself why there is a crack though. There is generally a reason. If its through movement of boards you will find it keeps coming back. It would need scrim or paper-tape before filling. If the plaster is blown then ideally you would need to knock the loose stuff off or at the very least rake out the crack so that its large before you fill it. Hope this helps. Good luck Andy