Gas mixture used in former kingdom (3)
I believe the answer is:
air
'gas mixture' is the definition.
(I've seen this before)
'used in former kingdom' is the wordplay.
'used in' means to look at the middle letters (the letters used on the inside of the word).
'former' becomes 'creator' (I've seen this in another clue).
'kingdom' becomes 'ir' (I am not sure about this - if you are sure you should believe this answer much more).
The middle of 'creator' is 'a'.
'a'+'ir'='AIR'
Can you help me to learn more?
(Other definitions for air that I've seen before include "Ventilate, give voice to" , "Musical composition" , "Lay" , "Substance breathed in" , "Ventilate or give expression to" .)