Perpetual cross installed outside (8)
I believe the answer is:
external
'outside' is the definition.
(synonyms)
'perpetual cross installed' is the wordplay.
'perpetual' becomes 'eternal' (synonyms).
'cross' becomes 'x' (X is cross-shaped).
'installed' indicates putting letters inside.
'eternal' placed around 'x' is 'EXTERNAL'.
(Other definitions for external that I've seen before include "Surface" , "Superficial" , "Coming from outside" , "Lying outside" , "abroad" .)