Sounds like an opening for part of a gun (6)
I believe the answer is:
breech
'for part of a gun' is the definition.
(I've seen this before)
'sounds like an opening' is the wordplay.
'sounds like' shows a homophone (sound like).
'an opening' becomes 'breach' (breach is a kind of opening).
'breach' sounds like 'BREECH'.
(Other definitions for breech that I've seen before include "Lower part of a gun" , "Back end of a gun barrel" , "part of Winchester" , "type of delivery" , "One's behind" .)