Deny having once called about the gun (6)
I believe the answer is:
negate
'deny' is the definition.
(I've seen this in other clues)
'once called about the gun' is the wordplay.
'once called' becomes 'nee' (I've seen this in another clue).
'about' indicates putting letters inside.
'the gun' becomes 'gat' (gat is a kind of gun).
'nee' going around 'gat' is 'NEGATE'.
'having' acts as a link.
(Other definitions for negate that I've seen before include "Render ineffective" , "Show to be false" , "Disprove or nullify" , "Cause to have no effect" , "Contradict" .)