What is the meaning of "pass well clear" in the ICAO right of way rules?
This is probably one of those rules which is deliberately a bit open to interpretation. For example, a glider pilot will be happy to pass under another glider when hill soaring at 40 knots much, much closer than they would be if cruising at 100 knots! Other pilots will probably use standard separation minima, for example 500 feet might be acceptable in the powered circuit but 1000 feet is allowed in certain airways.

SKYbrary suggests that "well clear" means 500 feet, but again points out that it might not be sufficient when using TCAS.

The implication is of course intended. If one is "well clear" by 20,000 feet there is no need to take avoiding action!