Battered cod boxed in air or gas (6,7)
I believe the answer is:
carbon dioxide
'gas' is the definition.
(thesaurus)
'battered cod boxed in air' is the wordplay.
'battered' indicates an anagram.
'cod'+'boxed'+'in'+'air'='codboxedinair'
'codboxedinair' is an anagram of 'CARBON DIOXIDE'.
'or' is the link.
(Other definitions for carbon dioxide that I've seen before include "Gas formed by breathing out" , "Gas for dry ice" , "This one might expire" , "gas from exhaust?" , "CO2" .)