Colt might be secured by this composer, on reflection (7)
I believe the answer is:
Here is my best explanation:
'colt might be secured by this' is the definition.
(colt is a type of gun)
'composer on reflection' is the wordplay.
'composer' becomes 'holst' (Gustav Holst).
'on' becomes 're' ('on' can be mean 'with reference to').
'reflection' shows that the letters should be reversed in order.
're' backwards is 'er'.
(Other definitions for holster that I've seen before include "Gun-holder", "Pistol-case", "It can carry 17 Ac", "Gun sheath", "Leather case", "Case for a gun", "case for the pistol", "put gun away", "Sheath for a handgun" and "Case for a pistol".)