In a well known case, they form a flight (6)

I believe the answer is:

stairs

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

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

(Other definitions for stairs that I've seen before include "Way between floors" , "A set of rising steps" , "Flights of steps" , "Ascending flight of steps" , "Interior means of ascent" .)

I've seen this clue in the Evening Standard.

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