Common gas (6)
'common gas' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for nitrogen that I've seen before include "Common gas, N" , "Gaseous element" , "Main element of air" , "Neon grit (anag.)" , "Element - atomic no. 7" .)
(Other definitions for hydrogen that I've seen before include "Lightest of all elements" , "With oxygen, it forms water" , "Combines chemically with oxygen to form water" , "Flammable gas" , "Bleach compound" .)
'common gas' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for oxygen that I've seen before include "Vital gas element" , "A combustible element" , "Life-supporting component of air" , "Gas that combines with hydrogen to form water" , "The second most common element on earth" .)