One in flight has tantrums when climbing (4)
I believe the answer is:
step
'one in flight' is the definition.
(part of a flight of stairs)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for step that I've seen before include "Pest (anag.)" , "Pace; foothold" , "Single pace" , "Move a little way" , "Pace for a pest" .)