Be a close thing! (6)

I believe the answer is:

adjoin

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'be' is the definition.
Although both the answer and definition are verbs in their base form, I don't understand how they can define each other.

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for adjoin that I've seen before include "Neighbour" , "Be next or nearest to" , "Be next to (something)" , "Meet - touch" , "Of building, be situated next to" .)

I've seen this clue in the Evening Standard.

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