Majestic Elgar composition (5)
I believe the answer is:
regal
'majestic' is the definition.
(I know that majestic can be written as regal)
'elgar composition' is the wordplay.
'composition' indicates anagramming the letters (letters arranged or composed in a different order).
'elgar' with letters rearranged gives 'REGAL'.
(Other definitions for regal that I've seen before include "Kingly or queenly" , "Portable organ" , "Befitting a monarch" , "fit for the Queen" , "Royal - 5 down (anag)" .)