You will shout (4)
I believe the answer is:
yell
'shout' is the definition.
(yelling is a kind of shouting)
'you will' is the wordplay.
'you' becomes 'ye' (**).
'will' becomes 'll'.
'ye'+'ll'='YELL'
(Other definitions for yell that I've seen before include "Give a sudden loud cry" , "Loud sharp cry" , "Clement --, politician" , "Scream" , "Shetland isle" .)