Abiding outside, with sign of something wrong inside and outside (8)
I believe the answer is:
external
'outside' is the definition.
(I know that outside can be written as external)
'abiding outside with sign of something wrong inside' is the wordplay.
'abiding outside with' becomes 'eternal' (I can't justify this - if you can you should believe this answer much more).
'sign of something wrong' becomes 'X' (used to mark an incorrect answer).
'inside' indicates putting letters inside.
'eternal' placed around 'x' is 'EXTERNAL'.
'and' is the link.
Can you help me to learn more?
(Other definitions for external that I've seen before include "Coming from outside" , "Belonging to the outside" , "Superficial" , "abroad" , "Relating to the outside" .)