Composer upset about pistol case (7)
I believe the answer is:
holster
'pistol case' is the definition.
(I've seen this before)
'composer upset about' is the wordplay.
'composer' becomes 'holst' (Gustav Holst).
'upset' is a reversal indicator.
'about' becomes 're' (regarding**).
're' back-to-front is 'er'.
'holst'+'er'='HOLSTER'
(Other definitions for holster that I've seen before include "Gun-holder" , "case for weapons" , "Revolver' cover" , "that carries a gun" , "armrest" .)