That man's hiding small sound of gas escaping (4)
I believe the answer is:
hiss
'sound of gas escaping' is the definition.
'that man's hiding small' is the wordplay.
'that man' becomes 'his' (I can't explain this - if you can you should give a lot more credence to this answer).
'hiding' indicates putting letters inside.
'small' becomes 's' (abbreviation - e.g. clothes size).
'his' placed around 's' is 'HISS'.
Can you help me to learn more?
(Other definitions for hiss that I've seen before include "showing contempt?" , "Jeer" , "Urgently whisper" , "Catcall" , "Snake's sibilant sound" .)