Majestic form of lager (5)
I believe the answer is:
regal
'majestic' is the definition.
(I know that majestic can be written as regal)
'form of lager' is the wordplay.
'form of' indicates anagramming the letters (the letters need to be in another 'form').
'lager' anagrammed gives 'REGAL'.
(Other definitions for regal that I've seen before include "Fit for a monarch" , "Imperial, majestic" , "King's" , "Portable organ" , "Befitting a monarch" .)