The efficient way
Stop work on the thing that is dependent until the dependency is removed. Deliver no value during this time. Learn nothing during this time. Start the work again when the dependency is removed.
This way is efficient as it only does work that will be kept, nothing gets thrown away.
The effective way
Separate the work from thing it is dependent on. Do the work in another way. Deliver value sooner. Learn more. Redo the work when the dependency is removed.
This way is effective as it enables users to get value and the team to learn.