It's slightly open, which gives one a shock (4)

I believe the answer is:

ajar

'it's slightly open' is the definition.
(as in a door left ajar)

'one a shock' is the wordplay.
'shock' becomes 'jar' (I've seen this before).
'a'+'jar'='AJAR'

'which gives' is the 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" .)

I've seen this clue in the Evening Standard.

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