Just fluorine and a mixture of gases (4)
I believe the answer is:
fair
'just' is the definition.
(just can mean fair in the sense of showing justice)
'fluorine and a mixture of gases' is the wordplay.
'fluorine' becomes 'F' (F is the chemical symbol for fluorine).
'and' is a charade indicator (letters next to each other).
'mixture of' is an anagram indicator (I've seen 'mixture' mean this).
'gases' becomes 'ir' (I am not sure about this - if you are sure you should believe this answer much more).
'f' after 'a' is 'af'.
'af' anagrammed gives 'fa'.
'fa'+'ir'='FAIR'
Can you help me to learn more?
(Other definitions for fair that I've seen before include "Decent" , "Women are known as the ... sex" , "Blonde or equitable" , "Just; passable" , "not cloudy" .)