Elementary arrangement for organ (5)
I believe the answer is:
argon
'elementary' is the definition.
(I've seen this before)
'arrangement for organ' is the wordplay.
'arrangement for' indicates anagramming the letters (I've seen this in other clues).
'organ' is an anagram of 'ARGON'.
(Other definitions for argon that I've seen before include "An insert gas to groan about" , "Chemical element" , "Inert gas used in light bulbs" , "Inert gas, present in small amounts in the air" , "Gas, 1% of atmosphere" .)