Crossword Genius

Happening to be tumbling (9)

Ross

I believe the answer is:

befalling

'happening to' is the definition.
('befall' can be a synonym of 'happen to')

'be tumbling' is the wordplay.
'tumbling' becomes 'falling' (I've seen this before).
'be'+'falling'='BEFALLING'

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