Jeer at that man’s son (4)
I believe the answer is:
hiss
'jeer' is the definition.
(I've seen this before)
'at that man's son' is the wordplay.
'at' is a charade indicator (letters next to each other).
'that man' becomes 'his' (I can't explain this - if you can you should believe this answer much more).
'son' becomes 's'.
'his' next to 's' is 'HISS'.
Can you help me to learn more?
(Other definitions for hiss that I've seen before include "(Make noise to) express disapproval" , "Urgent whisper" , "Snake's sibilant sound" , "Catcall" , "Wordless sound of disapproval" .)