It may not be plain where this route goes (8)
I believe the answer is:
highroad
'it may not be plain where this route goes' is the definition.
'highroad' can be an answer for 'route' (highroad is a kind of route). I'm unsure of the remainder of the definition.
'route goes' is the wordplay.
I cannot quite see how this works, but
'route' could be 'road' (synonyms) and 'road' is located in the answer.
The remaining letters 'high' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Another definition for highroad that I've seen is " Upper route?".)