About to adopt unfashionable course (5)
I believe the answer is:
route
'course' is the definition.
(both can mean a path)
'about to adopt unfashionable' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'to adopt' indicates putting letters inside (inserted letters are taken in or adopted).
'unfashionable' becomes 'out' (similar in meaning).
're' going around 'out' is 'ROUTE'.
(Other definitions for route that I've seen before include "Send a particular way" , "Way, course" , "Way, line" , "Course of 7" , "The way to get somewhere" .)