Open with a shock (4)
I believe the answer is:
ajar
'open' is the definition.
('ajar' can be similar in meaning to 'open')
'with a shock' is the wordplay.
'with' means one lot of letters go next to another.
'shock' becomes 'jar' (I've seen this in other clues).
'a' put next to 'jar' is 'AJAR'.
(Other definitions for ajar that I've seen before include "It's nearly open" , "partly opened" , "Not quite closed" , "letting in a little light?" , "Left open" .)