Blend including oxygen, unknown quantity in air (6)
I believe the answer is:
melody
'air' is the definition.
(I know that air can be written as melody)
'blend including oxygen unknown quantity' is the wordplay.
'blend' becomes 'meld' (synonyms).
'including' means one lot of letters goes inside another.
'oxygen' becomes 'O' (O is the chemical symbol for oxygen).
'unknown quantity' becomes 'y' (algebra).
'meld' placed around 'o' is 'melod'.
'melod'+'y'='MELODY'
'in' acts as a link.
(Other definitions for melody that I've seen before include "Refrain" , "My dole (anag.)" , "Sweet music, tuneful song" , "Agreeable succession of musical sounds" , "Musical theme" .)