Once having hidden the gun, deny it (6)
I believe the answer is:
negate
'gun deny it' is the definition.
I can't judge whether this definition defines the answer.
'once having hidden the' is the wordplay.
'once' becomes 'nee' (I've seen this in another clue).
'having' means one lot of letters goes inside another.
'hidden' becomes 'ga' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'ga'+'t'='gat'
'nee' placed around 'gat' is 'NEGATE'.
Can you help me to learn more?
(Other definitions for negate that I've seen before include "Show to be false" , "Cause to have no effect" , "Cancel; invalidate" , "Contradict" , "Make invalid" .)