Heads turned when English introduced weapon (4,3)
I believe the answer is:
sten gun
'weapon' is the definition.
(I've seen this before)
'heads turned when english introduced' is the wordplay.
'heads' becomes 'nuts' (I've seen this before).
'turned' is a reversal indicator.
'when english' becomes 'eng' (I am not sure about the 'when' bit.**).
'introduced' is an insertion indicator.
'nuts' backwards is 'stun'.
'stun' enclosing 'eng' is 'STEN GUN'.
(Other definitions for sten gun that I've seen before include "One fires" , "Killer" , "weapon" .)