Outside, shunning cross, but immortal (7)
I believe the answer is:
eternal
'immortal' is the definition.
(similar in meaning)
'outside shunning cross' is the wordplay.
'outside' becomes 'external' (similar in meaning).
'shunning' suggests deleting specific letters.
'cross' becomes 'x' (an X is a cross).
'external' with 'x' taken away is 'ETERNAL'.
'but' acts as a link.
(Other definitions for eternal that I've seen before include "Ceaseless" , "Never-ending, existing always" , "Perpetual" , "Forever, never-ending" , "One being forever" .)