Mistake concerning part of gun (9)
I believe the answer is:
oversight
'mistake' is the definition.
(oversight is a kind of mistake)
'concerning part of gun' is the wordplay.
'concerning' becomes 'over' (I've seen this before).
'part of gun' becomes 'sight' (the sight of a gun).
'over'+'sight'='OVERSIGHT'
(Other definitions for oversight that I've seen before include "Shot giver due to unintentional omission" , "Supervisor's charged with this" , "Carelessness" , "Supervision - omission" , "Neglect" .)