Old joke about bit of zombifying gas (5)
I believe the answer is:
ozone
'gas' is the definition.
(I've seen this before)
'old joke about bit of zombifying' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'joke' becomes 'one' (I've seen this before).
'about' is a charade indicator (letters next to each other).
'bit of' suggests taking the first letters.
The initial letter of 'zombifying' is 'z'.
'one' after 'z' is 'zone'.
'o'+'zone'='OZONE'
(Other definitions for ozone that I've seen before include "Ring" , "Sea air (found in layer?)" , "something found in the atmosphere" , "Condensed oxygen; sea air" , "Gas formed from oxygen" .)