Colt might be secured by this composer, on reflection (7)
I believe the answer is:
holster
'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'.
'holst'+'er'='HOLSTER'
(Other definitions for holster that I've seen before include "Pistol-holder" , "Holder for a hand-gun" , "Receptacle for a hand-gun" , "put gun away" , "armrest" .)