Indestructible cross nicked from outside (7)
I believe the answer is:
eternal
'indestructible' is the definition.
(thesaurus)
'cross nicked from outside' is the wordplay.
'cross' becomes 'x' (X is cross-shaped).
'nicked from' is a deletion indicator.
'outside' becomes 'external' (synonyms).
'external' with 'x' taken away is 'ETERNAL'.
(Other definitions for eternal that I've seen before include "Timeless" , "Forever and always" , "Forever or unchanging" , "Never failing" , "carrying on endlessly" .)