To what extent was the Suez Crisis a start of decolonisation of the British Empire in Africa?
The Suez War was one of the first episodes in the decolonization of Africa. However, it was not really the cause of that decolonization. Instead, both the Suez Crisis and the decolonization of Africa were symptoms of Britain's loss of world power and prestige.
The main causes of the decolonization of Africa were the rise of US power and the Cold War. Both of these came out of WWII. After WWII, Britain was no longer a dominant world power. It was greatly reduced in terms of its military and economic powers relative to countries like the US and the USSR. This loss of power led to the decolonization of India. It also led to the British being forced to back down in the Suez Crisis. Finally, it led to Britain being pressured to decolonize all of its African empire.
In this way, the Suez Crisis did happen early in the decolonization process, but it was not the cause of that process.