Good as neon or helium (3)
I believe the answer is:
gas
'neon or helium' is the definition.
(I've seen this in another clue)
'good as' is the wordplay.
'good' becomes 'g' (abbreviation).
'g'+'as'='GAS'
(Other definitions for gas that I've seen before include "Empty talk (colloq.)" , "Eg, oxygen" , "Hydrogen or methane, say" , "Form of fossil fuel" , "Neon, for example" .)