Flight school hurdle (4)
I believe the answer is:
solo
'flight school hurdle' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for solo that I've seen before include "Performance by one person" , "Game for four" , "something for one to do" , "Piece of music for one" , "Unaccompanied passage" .)