Peripheral route, possibly (5)
I believe the answer is:
outer
'peripheral' is the definition.
(I've seen this before)
'route possibly' is the wordplay.
'possibly' is an anagram indicator.
'route' with letters rearranged gives 'OUTER'.
(Other definitions for outer that I've seen before include "Exterior or external, like space" , "External kind of route" , "This space is beyond earth's atmosphere" , "External; peripheral" , "On the periphery" .)