Organ’s playing — a gas! (5)
I believe the answer is:
argon
'a gas' is the definition.
(argon is a kind of gas)
'organ's playing' is the wordplay.
'playing' indicates an anagram.
'organ' anagrammed gives 'ARGON'.
(Other definitions for argon that I've seen before include "something inert and insubstantial" , "Inert gas, present in small amounts in the air" , "rare gas" , "noble element" , "Noble gas, atomic number 18" .)