Sound of gas escaping that man's son (4)
I believe the answer is:
hiss
'sound of gas escaping' is the definition.
'man's son' is the wordplay.
'man' becomes 'his' (I've seen this in other clues).
'son' becomes 's' (genealogical abbreviation for son).
'his'+'s'='HISS'
'that' is the link.
(Other definitions for hiss that I've seen before include "(Make noise to) express disapproval" , "Sharp sibilant sound" , "Snake noise" , "Boo" , "Make sibilant sound of disapproval as at villain" .)