Case of composer about to retire (7)
I believe the answer is:
holster
'case' is the definition.
(I've seen this before)
'composer about to retire' is the wordplay.
'composer' becomes 'holst' (Gustav Holst).
'about' becomes 're' (regarding).
'to retire' says the letters should be written in reverse (retire can mean to go back).
're' in reverse letter order is 'er'.
'holst'+'er'='HOLSTER'
'of' is the link.
(Other definitions for holster that I've seen before include "Gun holder" , "Holder for a hand-gun" , "Gun case" , "Lost her (anag.)" , "Revolver' cover" .)