Rubber silencer fitted round American gun (6)
I believe the answer is:
sheath
'rubber' is the definition.
(I know that rubber can be written as sheath)
'silencer fitted round american gun' is the wordplay.
'silencer' becomes 'sh' ('sh' is used to demand silence).
'fitted round' means one lot of letters goes inside another.
'american gun' becomes 'heat' (as in 'packing heat').
'sh' enclosing 'heat' is 'SHEATH'.
(Other definitions for sheath that I've seen before include "cocktail dress" , "Case for a dagger" , "Sword covering" , "Scabbard" , "case for weapon" .)