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' (chemical symbol).
'and' means one lot of letters go next to another.
'mixture of' indicates an anagram (I've seen 'mixture' mean this).
'gases' becomes 'ir' (I can't explain this - if you can you should believe this answer much more).
'f' put after 'a' is 'af'.
'af' with letters rearranged 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" .)