Immortal unknown's left outside (7)
I believe the answer is:
eternal
'immortal' is the definition.
(similar in meaning)
'unknown's left outside' is the wordplay.
'unknown' becomes 'x' (algebra).
'left' indicates named letters should be taken away.
'outside' becomes 'external' (similar in meaning).
'external' with 'x' removed is 'ETERNAL'.
(Other definitions for eternal that I've seen before include "Existing forever" , "carrying on endlessly" , "Valid for all time" , "Infinite - endless" , "non-stop" .)