Crossword Genius

Befallen without mishap (4)

Ross

I believe the answer is:

been

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'befallen' is the definition.
Although both the answer and definition are past participle verbs, I cannot understand how they can define each other.

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for been that I've seen before include "Past participle of be" , "(Have) gone in the past (to somewhere)" .)

I've seen this clue in the Evening Standard.
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