I suspect that you will find that it is not well defined in ICAO. In the UK the CAA issued an
ATSIN which said that individual airports might want to define what constituted clear of the runway - but sadly the document has been withdrawn and I don't think the content has been published anywhere else. Maybe if you contact the CAA they can dig out a copy for you but I don't think it gives you a definitive answer.
ICAO gives a hint in a recent amendment to PANS-ATM (and I think this was new in the amendment) - references to runway vacated reports are quite specific as below:
7.5.3.1.2.2 If the control tower is unable to determine, either visually or by radar, that a vacating or crossing aircraft has cleared the runway, the aircraft shall be requested to report when it has vacated the runway. The report shall be made when the entire aircraft is beyond the relevant runway-holding position.
By inference, an aircraft will have 'entered' a runway when any part of it has crossed the runway-holding position.