Flight instruments I replaced (6)

I believe the answer is:

stairs

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'flight' is the definition.
(I've seen this in another clue)

I cannot understand how the rest of the clue works.

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

I've seen this clue in The Guardian.

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