Concerning about impossible flight path (5)
I believe the answer is:
route
'flight path' is the definition.
(thesaurus)
'concerning about impossible' is the wordplay.
'concerning' becomes 're' (both can mean on a particular topic).
'about' means one lot of letters goes inside another.
'impossible' becomes 'out' (out can mean no longer possible).
're' enclosing 'out' is 'ROUTE'.
(Other definitions for route that I've seen before include "Course taken" , "Course of journey" , "track" , "Way, line" , "Course of 7" .)