A container slightly open (4)

I believe the answer is:

ajar

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

'a container' is the wordplay.
'container' becomes 'jar' (jar is a kind of container).
'a'+'jar'='AJAR'

(Other definitions for ajar that I've seen before include "Not quite closed" , "Barely open" , "Unlatched" , "Mourn" , "Half-open" .)

I've seen this clue in The Sun, The Times and The Guardian.

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