Gun carrier a composer's taken to Her Majesty (7)
I believe the answer is:
holster
'gun carrier' is the definition.
'a composer's taken to her majesty' is the wordplay.
'a composer' becomes 'holst' (Gustav Holst).
'taken' is a charade indicator (letters next to each other).
'to her majesty' becomes 'ER' (Elizabeth Regina).
'holst'+'er'='HOLSTER'
(Other definitions for holster that I've seen before include "Pistol sheath" , "Gun case" , "Leather case" , "Gun-holder" , "armrest" .)