The perimeter route, possibly (5)
I believe the answer is:
outer
'the perimeter route possibly' is the definition.
I can't tell whether this definition defines the answer.
'route possibly' is the wordplay.
'possibly' is an anagram indicator.
'route' is an anagram of 'OUTER'.
Can you help me to learn more?
(Other definitions for outer that I've seen before include "Poor shot" , "Ring for double scores on a dartboard" , "Ring furthest from the bullseye" , "Surrounding" , "External; part of archery target" .)