It has to be negotiated in flight (5)
I believe the answer is:
'flight' is the definition.
(I've seen this before)
'it has to be negotiated' is the wordplay.
I cannot quite see how this works, but
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found in the answer.
The remaining letters 'sair' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'in' is the link.
Can you help me to learn more?
(Other definitions for stair that I've seen before include "Support on which to step" , "Flight unit" , "Set of steps" , "One of a 15" , "one on the way up" .)