The route they are following is rough, I hear (6)
I believe the answer is:
course
'the route' is the definition.
(course is a kind of route)
'rough i hear' is the wordplay.
'rough' becomes 'coarse' (similar in meaning).
'i hear' indicates a 'sounds like' (homophone) clue.
'coarse' is a homophone of 'COURSE'.
'they are following is' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for course that I've seen before include "Routine progress" , "Venue for race or golf tournament" , "Syllabus" , "Source (anag.)" , "Direction; lecture series" .)