APMR
In the scenario you paint, the correct read back,assuming one hasn't heard the "when ready" is; 7,000, left/leaving [cruise level], ABC
Just responding to a level assignment doesn't indicate the departure from the old level, hence the requirement to report left/leaving a maintained level.
Occasionally in the confusion, ATC will omit the usual "when ready", or we will miss the "when ready". If unsure it's simple to get clarification.