Oxygen, for one (3)
'oxygen for one' is the definition.
(oxygen is a kind of element)
This is all the clue.
(Other definitions for element that I've seen before include "part of cooker" , "Constituent of universe" , "Constituent part (7)" , "part of a toaster" , "Fundamental part of whole" .)
'oxygen for one' is the definition.
(oxygen is a kind of gas)
This is the entire clue.
(Other definitions for gas that I've seen before include "Americans' petrol" , "Chatter (colloq.)" , "Nitrogen, say" , "What's needed to drive" , "Petrol (US)" .)