Deny once having carried a gun (6)
I believe the answer is:
negate
'deny' is the definition.
(negate can mean to deny or refute)
'having carried a gun' is the wordplay.
'having' becomes 'nee' (I can't justify this - if you can you should believe this answer much more).
'carried' means one lot of letters goes inside another.
'a gun' becomes 'gat' (US informal term for a gun).
'nee' placed around 'gat' is 'NEGATE'.
'once' is the link.
Can you help me to learn more?
(Other definitions for negate that I've seen before include "Age ten (anag)" , "Countermand" , "Deny the truth of" , "Discredit" , "Render null or disprove" .)