OAP: If in doubt, for God's sake ask!!! Even if it's on the R/T. We don't mind, honest.
That's easy to say coracle, but R/T loading is already close to capacity on some of the LTCC frequencies. If every aircraft queries the initial instruction because the interpretation is ambiguous, you're going to make things a lot worse.
If a pilot has to question the meaning of an instruction or clearance, the communication procedure has failed somewhere. There is no room for ambiguity.
Unfortunately there is no document that tells pilots how to interpret clearances or particular phrasings of clearances.