Having tantrums on retreat, one in flight (4)
I believe the answer is:
step
'one in flight' is the definition.
(part of a flight of stairs)
I cannot understand how the rest of the clue works.
(Other definitions for step that I've seen before include "bit of a dance" , "Place foot on as with a pest" , "Football" , "A pace for pets" , "Gradation" .)