Iodine and oxygen not specified as particles formed in gas (4)
I believe the answer is:
ions
'particles formed in gas' is the definition.
I can't tell whether this definition defines the answer.
'iodine and oxygen not specified' is the wordplay.
'iodine' becomes 'I' (chemical symbol).
'and' is a charade indicator (letters next to each other).
'oxygen' becomes 'O' (atomic symbol for oxygen).
'not specified' becomes 'ns' (this could be a standard abbreviation which I don't know about).
'i'+'o'+'ns'='IONS'
'as' acts as a link.
Can you help me to learn more?
(Other definitions for ions that I've seen before include "items with a charge" , "N in radio comms." , "Electrically-charged particles" , "Electrically charged particles" , "Charged atom or molecule" .)