Gas, one removed from air, almost disappeared (5)
I believe the answer is:
argon
'gas' is the definition.
(argon is a gas)
'air almost disappeared' is the wordplay.
'air' becomes 'ar' (I am not sure about this - if you are sure you should believe this answer much more).
'almost' means to remove the last letter (almost the whole word is kept).
'disappeared' becomes 'gone' (going is a kind of disappearing).
'gone' with its final letter taken away is 'gon'.
'ar'+'gon'='ARGON'
'one removed from' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for argon that I've seen before include "17 [GAS]" , "Gas, inert constituent of air" , "noble element" , "Inert gas in atmosphere" , "Noble gas used in light bulbs" .)