opnot
I think you are referring to "cleared immediate take-off" rather than "expedite".
Incedentally, my dog-eared old MATS Part 1 states in itallics on E(Attach)-3
Expedite climb/descent
To require a pilot to climb/descend at best rate
So despite what I do in reality, MATS Part 1 does define what the term means, even if it is not what you actually want to achieve!
G W-H