Gallium, sulphur or helium, perhaps? (3)
I believe the answer is:
gas
'helium perhaps?' is the definition.
(helium is a kind of gas)
'gallium sulphur' is the wordplay.
'gallium' becomes 'Ga' (Ga is the chemical symbol for gallium).
'sulphur' becomes 'S' (synonyms).
'ga'+'s'='GAS'
'or' acts as a link.
(Other definitions for gas that I've seen before include "Hydrogen possibly" , "In brief, American petrol" , "Talk idly; fuel" , "Talk profusely" , "Gossip" .)