Crossword Genius

How can a door be this? (4)

Ross

I believe the answer is:

ajar

I'm a little stuck... Click here to teach me more about this clue!

'door be this?' is the definition.
The definition suggests a singular noun but the answer is not.

'how can a' is the wordplay.
I cannot really understand how this works, but
'a' is present in the answer.
The remaining letters 'jar' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

(Other definitions for ajar that I've seen before include "Partly open" , "It's nearly open" , "A little bit open, like door" , "Not to answer" , "Mourn" .)

I've seen this clue in the Evening Standard.
Want a hint initially instead of a full solution? Install my app