Course, unfashionable, in Religious Education (5)
I believe the answer is:
route
'course' is the definition.
(both can mean a path)
'unfashionable in religious education' is the wordplay.
'unfashionable' becomes 'out' (similar in meaning).
'in' indicates putting letters inside.
'religious education' becomes 'RE' (abbreviation).
'out' going inside 're' is 'ROUTE'.
(Other definitions for route that I've seen before include "Course to destination" , "Established line of trave" , "Direction of travel" , "Way to travel" , "Way, line" .)