When the TARDIS enters siege mode Clara is literally cut off from the voice of the Doctor. Yet these two have spent the entire series walling themselves off from each other. Miscommunication, willful and otherwise, is an ongoing theme between all three of the main parties involved. Danny and Clara keep secrets from each other and Clara and the Doctor talk at rather to one another. It makes one wonder if the TARDIS planned this trip as a kind of warning. In Mummy on the Orient Express the Doctor was forced to surrender to the Mummy to end the siege of the train. In Flatline he must surrender control to Clara. This has mixed results. While Clara emerges as a strong leader she is drawn to the addiction of power. In a sense her morality is under siege. She can choose to hold her ground and attempt to find some sort of balance or give up her ethical center in a quest for control.