Concerning about unfashionable course (5)
I believe the answer is:
route
'course' is the definition.
(both can mean a path)
'concerning about unfashionable' is the wordplay.
'concerning' becomes 're' (both can mean on a particular topic).
'about' means one lot of letters goes inside another.
'unfashionable' becomes 'out' (synonyms).
're' enclosing 'out' is 'ROUTE'.
(Other definitions for route that I've seen before include "Path to follow" , "Course taken" , "Course to be travelled" , "Course to destination" , "Way, course" .)