You will scream (4)
I believe the answer is:
yell
'scream' is the definition.
(yell can mean to scream or cry out)
'you will' is the wordplay.
'you' becomes 'ye' (historical version of 'you').
'will' becomes ''ll' (contracted form, as in 'he'll' - 'he will').
'ye'+'ll'='YELL'
(Other definitions for yell that I've seen before include "Howl" , "Shetland isle" , "Loud utterance" , "Shout loudly" , "Shout - Shetland island" .)