Backbone injected with liquid initially, or gas (8)
I believe the answer is:
chlorine
'gas' is the definition.
(I know that chlorine is a type of gas)
'backbone injected with liquid initially or' is the wordplay.
'backbone' becomes 'chine' (chine is a kind of backbone).
'injected with' means one lot of letters goes inside another.
'initially' indicates taking the first letters.
The first letter of 'liquid' is 'l'.
'l'+'or'='lor'
'chine' going around 'lor' is 'CHLORINE'.
(Other definitions for chlorine that I've seen before include "It's used to purify water" , "Chemical used as poison gas and water supply disinfectant" , "Pale green gaseous element" , "Gas used in STERILISING water and in industry (8)" , "Pale green gas" .)