Hydrogen gas shock (4)
I believe the answer is:
hair
'shock' is the definition.
(I've seen this before)
'hydrogen gas' is the wordplay.
'hydrogen' becomes 'h' ('H' can be a synonym of 'hydrogen').
'gas' becomes 'air'.
'h'+'air'='HAIR'
(Other definitions for hair that I've seen before include "Mane" , "Musical with nude scenes" , "Whisker" , "The American Tribal Love-Rock Musical (1967)" , "Outgrowth from the skin" .)