Sat back with the queen's stun gun (5)
I believe the answer is:
taser
'stun gun' is the definition.
(I've seen this before)
'sat back with the queen's' is the wordplay.
'back' is a reversal indicator.
'with' says to put letters next to each other.
'the queen' becomes 'ER' (abbreviation for Elizabeth Regina).
'sat' written backwards gives 'tas'.
'tas'+'er'='TASER'
(Other definitions for taser that I've seen before include "A temporary stunner?" , "Police stun weapon" , "Device used to stun someone temporarily" , "The weapon delivers a temporary paralysing electric shock" , "Stunning weapon" .)