Going on for ever about unidentified person outside (8)
I believe the answer is:
external
'outside' is the definition.
(synonyms)
'going on for ever about unidentified person' is the wordplay.
'going on for ever' becomes 'eternal' (I've seen this before).
'about' indicates putting letters inside.
'unidentified person' becomes 'x' (X can represent an unknown person).
'eternal' going around 'x' is 'EXTERNAL'.
(Other definitions for external that I've seen before include "Outward" , "abroad" , "Superficial" , "Belonging to the outside" , "Outer" .)