Confirmation of right to hold gun overturned (7)
I believe the answer is:
receipt
'confirmation' is the definition.
(I have seen 'Confirmation of purchase' mean 'receipt' so perhaps 'confirmation' could also mean 'receipt')
'right to hold gun overturned' is the wordplay.
'right' becomes 'rt' (abbreviation as in Rt Hon- the Right Honourable).
'to hold' indicates putting letters inside.
'gun' becomes 'piece' (piece is a kind of gun).
'overturned' shows that the letters should be reversed in order.
'piece' back-to-front is 'eceip'.
'rt' placed around 'eceip' is 'RECEIPT'.
'of' acts as a link.
(Other definitions for receipt that I've seen before include "Acceptance" , "Sales slip" , "Confirmation of purchase" , "Written acknowledgment of payment" , "Proof of purchase" .)