Old cardinal inhaling unknown gas (5)
I believe the answer is:
ozone
'gas' is the definition.
(I know that ozone is a type of gas)
'old cardinal inhaling unknown' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'cardinal' becomes 'one' (a cardinal number).
'inhaling' indicates putting letters inside.
'unknown' becomes 'z' (algebra).
'o'+'one'='oone'
'oone' placed around 'z' is 'OZONE'.
(Other definitions for ozone that I've seen before include "By sea, you could take in this" , "Form of atmospheric oxygen" , "Form of oxygen in earth's upper atmosphere - there's a hole in it" , "A poisonous gas; seaside air" , "Trioxygen" .)