Gold metal reflected information identifying gas (8)
I believe the answer is:
nitrogen
'gas' is the definition.
(I know that nitrogen is a type of gas)
'gold metal reflected information' is the wordplay.
'gold' becomes 'or' ('or' is the heraldic name for gold).
'metal' becomes 'tin' (tin is a kind of metal).
'reflected' says the letters should be written backwards (letters reflected back).
'information' becomes 'gen' (gen can informally mean information).
'or'+'tin'='ortin'
'ortin' written backwards gives 'nitro'.
'nitro'+'gen'='NITROGEN'
'identifying' acts as a link.
(Other definitions for nitrogen that I've seen before include "It constitutes 78 percent of the atmosphere" , "Gas forming about four fifths of the atmosphere" , "chemists have fixation with?" , "It makes up 78% of air" , "Chemical element" .)