Criminal three packing a gun once in America (6)
I believe the answer is:
heater
'gun once in america' is the definition.
'heater' can be an answer for 'gun' (I've seen this in another clue). I'm not sure about the 'once in america' bit.
'criminal three packing a' is the wordplay.
'criminal' indicates an anagram.
'packing' means one lot of letters goes inside another (some letters are packed inside).
'three' is an anagram of 'heter'.
'heter' enclosing 'a' is 'HEATER'.
Can you help me to learn more?
(Other definitions for heater that I've seen before include "Radiator" , "Source of warmth; gun" , "Warming device" , "Room warmer" , "fire" .)