Instrument using argon (5)
I believe the answer is:
organ
'instrument' is the definition.
(I know that organ is a type of instrument)
'using argon' is the wordplay.
'using' indicates anagramming the letters (the letters used in a different way).
'argon' anagrammed gives 'ORGAN'.
(Other definitions for organ that I've seen before include "Ear, perhaps" , "ear maybe" , "with a lofty tone?" , "Agency" , "May be in the loft" .)