A vessel slightly open (4)

I believe the answer is:

ajar

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

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

(Other definitions for ajar that I've seen before include "A little bit open, like door" , "It's nearly open" , "Barely open" , "opened tentatively?" , "(Of door) slightly open" .)

I've seen this clue in the Evening Standard.

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