It's slightly open, which gives one a shock (4)
I believe the answer is:
ajar
'it's slightly open' is the definition.
(I've seen this in another clue)
'one a shock' is the wordplay.
'shock' becomes 'jar' (both can mean to shake).
'a'+'jar'='AJAR'
'which gives' acts as a link.
(Other definitions for ajar that I've seen before include "A litle bit open" , "A little bit open, like door" , "It's nearly open" , "Partly open" , "Fractionally open" .)