Furthest sort of route (5)
I believe the answer is:
outer
'furthest' is the definition.
Both the definition and answer are adjectives. Maybe you can see an association between them that I don't see?
'sort of route' is the wordplay.
'sort of' indicates an anagram.
'route' with letters rearranged gives 'OUTER'.
Can you help me to learn more?
(Other definitions for outer that I've seen before include "Exterior or external, like space" , "Poor shot" , "Route (anag.)" , "Ring for double scores on a dartboard" , "This space is beyond earth's atmosphere" .)