External route revised (5)
I believe the answer is:
outer
'external' is the definition.
('outer' can be similar in meaning to 'external')
'route revised' is the wordplay.
'revised' indicates anagramming the letters.
'route' is an anagram of 'OUTER'.
(Other definitions for outer that I've seen before include "Archer's shot" , "This space is beyond earth's atmosphere" , "Surrounding" , "On the periphery" , "Route (anag.)" .)