Dodo could never manage this many stairs (6)
I believe the answer is:
flight
I believe this clue is a double definition.
'dodo could never manage this' is the first definition.
'many stairs' is the second definition.
(flight of stairs)
(Other definitions for flight that I've seen before include "Stairs; escape" , "Group of birds - running away" , "Aviation" , "Series of steps" , "Air transport - running away" .)