Once you are going to cry (4)
I believe the answer is:
yell
'cry' is the definition.
(synonyms)
'once you are going to' is the wordplay.
'once you' becomes 'ye' (historical version of 'you'. I am not sure about the 'once' bit.).
'are going to' becomes 'll' (as in he'll, we'll etc).
'ye'+'ll'='YELL'
Can you help me to learn more?
(Other definitions for yell that I've seen before include "Shetland isle" , "Scream" , "Loud cry; rhymes with collapsed" , "Clement --, politician" , "Utter a sudden loud cry" .)