Shock, seizing engineer's weapon (4,3)
I believe the answer is:
sten gun
'weapon' is the definition.
(I've seen this before)
'shock seizing engineer's' is the wordplay.
'shock' becomes 'stun' (both can mean to surprise).
'seizing' indicates putting letters inside (inserted letters are seized).
'engineer' becomes 'eng' (abbreviation).
'stun' placed around 'eng' is 'STEN GUN'.
(Other definitions for sten gun that I've seen before include "Killer" , "weapon" , "One fires" .)