Combine yttrium with oxygen in air (6)
I believe the answer is:
melody
'air' is the definition.
(synonyms)
'combine yttrium with oxygen' is the wordplay.
'combine' becomes 'meld' (synonyms).
'yttrium' becomes 'Y' (chemical symbol).
'with' is an insertion indicator.
'oxygen' becomes 'O' (O is the chemical symbol for oxygen).
'meld'+'y'='meldy'
'meldy' going around 'o' is 'MELODY'.
'in' acts as a link.
(Other definitions for melody that I've seen before include "It forms the core of a tune" , "Air, tune" , "A pleasant tune or song" , "Sweet music, tuneful song" , "Agreeable succession of musical sounds" .)