Partially open a beer (4)

I believe the answer is:

ajar

'partially open' is the definition.
(I've seen this before)

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

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

I've seen this clue in The Telegraph.

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