Unaccompanied flight (4)
I believe the answer is:
solo
'unaccompanied flight' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for solo that I've seen before include "Unaccompanied passage" , "Alone, unaccompanied" , "Fly aircraft" , "Whist variant" , "something for one to do" .)