You say why you said following kid is known initially to be from East Africa (6)

'you say why' is the definition.
I can't judge whether this definition defines the answer.

'you said following kid is known initially to be from east africa' is the wordplay.
'said' shows a homophone (sound like).
'following' says to put letters next to each other.
'initially' says to take the initial letters.
'to be from east africa' becomes 'yu' (I can't justify this - if you can you should believe this answer much more).
The initial letters of 'kid is known' is 'kik'.
'you' is a homophone of 'u'.
'u' after 'kik' is 'kiku'.
'kiku'+'yu'='KIKUYU'

Can you help me to learn more?