Shout, “Start to block person who has lost his head” (6)

'shout start to block person who has lost his head' is the definition.
'bellow' can be an answer for 'shout' (bellow is a kind of shout). I'm not sure about the rest of the definition.

'shout start to block person who has lost' is the wordplay.
'shout' indicates a 'sounds like' (homophone) clue (I've seen 'shouted' mean this (how the word might sound if shouted)).
'start to' suggests taking the first letters.
'person who' becomes 'low' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'has' indicates putting letters inside.
'lost' becomes 'l' (abbreviation in sporting league tables).
The first letter of 'block' is 'b'.
'b' is a homophone of 'be'.
'low' placed around 'l' is 'llow'.
'be'+'llow'='BELLOW'

Can you help me to learn more?